diff --git a/calendar/lib.php b/calendar/lib.php
index 90b5f4057b7..929e82ae81a 100644
--- a/calendar/lib.php
+++ b/calendar/lib.php
@@ -728,85 +728,164 @@ function calendar_add_event_metadata($event) {
  * @param boolean $ignorehidden whether to select only visible events or all events
  * @return array $events of selected events or an empty array if there aren't any (or there was an error)
  */
-function calendar_get_events($tstart, $tend, $users, $groups, $courses, $withduration=true, $ignorehidden=true) {
+function calendar_get_events($tstart, $tend, $users, $groups, $courses, $withduration = true, $ignorehidden = true) {
     global $DB;
 
-    $whereclause = '';
     $params = array();
     // Quick test.
     if (empty($users) && empty($groups) && empty($courses)) {
         return array();
     }
 
+    // Array of filter conditions. To be concatenated by the OR operator.
+    $filters = [];
+
+    // User filter.
     if ((is_array($users) && !empty($users)) or is_numeric($users)) {
         // Events from a number of users
-        if(!empty($whereclause)) $whereclause .= ' OR';
         list($insqlusers, $inparamsusers) = $DB->get_in_or_equal($users, SQL_PARAMS_NAMED);
-        $whereclause .= " (e.userid $insqlusers AND e.courseid = 0 AND e.groupid = 0)";
+        $filters[] = "(e.userid $insqlusers AND e.courseid = 0 AND e.groupid = 0)";
         $params = array_merge($params, $inparamsusers);
-    } else if($users === true) {
+    } else if ($users === true) {
         // Events from ALL users
-        if(!empty($whereclause)) $whereclause .= ' OR';
-        $whereclause .= ' (e.userid != 0 AND e.courseid = 0 AND e.groupid = 0)';
-    } else if($users === false) {
-        // No user at all, do nothing
+        $filters[] = "(e.userid != 0 AND e.courseid = 0 AND e.groupid = 0)";
     }
+    // Boolean false (no users at all): We don't need to do anything.
 
+    // Group filter.
     if ((is_array($groups) && !empty($groups)) or is_numeric($groups)) {
         // Events from a number of groups
-        if(!empty($whereclause)) $whereclause .= ' OR';
         list($insqlgroups, $inparamsgroups) = $DB->get_in_or_equal($groups, SQL_PARAMS_NAMED);
-        $whereclause .= " e.groupid $insqlgroups ";
+        $filters[] = "e.groupid $insqlgroups";
         $params = array_merge($params, $inparamsgroups);
-    } else if($groups === true) {
+    } else if ($groups === true) {
         // Events from ALL groups
-        if(!empty($whereclause)) $whereclause .= ' OR ';
-        $whereclause .= ' e.groupid != 0';
+        $filters[] = "e.groupid != 0";
     }
-    // boolean false (no groups at all): we don't need to do anything
+    // Boolean false (no groups at all): We don't need to do anything.
 
+    // Course filter.
     if ((is_array($courses) && !empty($courses)) or is_numeric($courses)) {
-        if(!empty($whereclause)) $whereclause .= ' OR';
         list($insqlcourses, $inparamscourses) = $DB->get_in_or_equal($courses, SQL_PARAMS_NAMED);
-        $whereclause .= " (e.groupid = 0 AND e.courseid $insqlcourses)";
+        $filters[] = "(e.groupid = 0 AND e.courseid $insqlcourses)";
         $params = array_merge($params, $inparamscourses);
     } else if ($courses === true) {
         // Events from ALL courses
-        if(!empty($whereclause)) $whereclause .= ' OR';
-        $whereclause .= ' (e.groupid = 0 AND e.courseid != 0)';
+        $filters[] = "(e.groupid = 0 AND e.courseid != 0)";
     }
 
     // Security check: if, by now, we have NOTHING in $whereclause, then it means
     // that NO event-selecting clauses were defined. Thus, we won't be returning ANY
     // events no matter what. Allowing the code to proceed might return a completely
     // valid query with only time constraints, thus selecting ALL events in that time frame!
-    if(empty($whereclause)) {
+    if (empty($filters)) {
         return array();
     }
 
-    if($withduration) {
-        $timeclause = '(e.timestart >= '.$tstart.' OR e.timestart + e.timeduration > '.$tstart.') AND e.timestart <= '.$tend;
-    }
-    else {
-        $timeclause = 'e.timestart >= '.$tstart.' AND e.timestart <= '.$tend;
-    }
-    if(!empty($whereclause)) {
-        // We have additional constraints
-        $whereclause = $timeclause.' AND ('.$whereclause.')';
-    }
-    else {
-        // Just basic time filtering
-        $whereclause = $timeclause;
-    }
+    // Build our clause for the filters.
+    $filterclause = implode(' OR ', $filters);
 
+    // Array of where conditions for our query. To be concatenated by the AND operator.
+    $whereconditions = ["($filterclause)"];
+
+    // Time clause.
+    if ($withduration) {
+        $timeclause = "((e.timestart >= :tstart1 OR e.timestart + e.timeduration > :tstart2) AND e.timestart <= :tend)";
+        $params['tstart1'] = $tstart;
+        $params['tstart2'] = $tstart;
+        $params['tend'] = $tend;
+    } else {
+        $timeclause = "(e.timestart >= :tstart AND e.timestart <= :tend)";
+        $params['tstart'] = $tstart;
+        $params['tend'] = $tend;
+    }
+    $whereconditions[] = $timeclause;
+
+    // Show visible only.
     if ($ignorehidden) {
-        $whereclause .= ' AND e.visible = 1';
+        $whereconditions[] = "(e.visible = 1)";
     }
 
+    // Build the main query's WHERE clause.
+    $whereclause = implode(' AND ', $whereconditions);
+
+    // Build SQL subquery and conditions for filtered events based on priorities.
+    $subquerywhere = '';
+    $subqueryconditions = [];
+
+    // Get the user's courses. Otherwise, get the default courses being shown by the calendar.
+    $usercourses = calendar_get_default_courses();
+
+    // Set calendar filters.
+    list($usercourses, $usergroups, $user) = calendar_set_filters($usercourses, true);
+    $subqueryparams = [];
+
+    // Flag to indicate whether the query needs to exclude group overrides.
+    $viewgroupsonly = false;
+
+    if ($user) {
+        // Set filter condition for the user's events.
+        $subqueryconditions[] = "(ev.userid = :user AND ev.courseid = 0 AND ev.groupid = 0)";
+        $subqueryparams['user'] = $user;
+
+        foreach ($usercourses as $courseid) {
+            if (has_capability('moodle/site:accessallgroups', context_course::instance($courseid))) {
+                $usergroupmembership = groups_get_all_groups($courseid, $user, 0, 'g.id');
+                if (count($usergroupmembership) == 0) {
+                    $viewgroupsonly = true;
+                    break;
+                }
+            }
+        }
+    }
+
+    // Set filter condition for the user's group events.
+    if ($usergroups === true || $viewgroupsonly) {
+        // Fetch group events, but not group overrides.
+        $subqueryconditions[] = "(ev.groupid != 0 AND ev.eventtype = 'group')";
+    } else if (!empty($usergroups)) {
+        // Fetch group events and group overrides.
+        list($inusergroups, $inusergroupparams) = $DB->get_in_or_equal($usergroups, SQL_PARAMS_NAMED);
+        $subqueryconditions[] = "(ev.groupid $inusergroups)";
+        $subqueryparams = array_merge($subqueryparams, $inusergroupparams);
+    }
+
+    // Set filter condition for the user's courses.
+    if (!empty($usercourses)) {
+        list($inusercourses, $inusercoursesparams) = $DB->get_in_or_equal($usercourses, SQL_PARAMS_NAMED);
+        $subqueryconditions[] = "(ev.groupid = 0 AND ev.courseid $inusercourses)";
+        $subqueryparams = array_merge($subqueryparams, $inusercoursesparams);
+    }
+
+    // Build the WHERE condition for the sub-query.
+    if (!empty($subqueryconditions)) {
+        $subquerywhere = 'WHERE ' . implode(" OR ", $subqueryconditions);
+    }
+
+    // Merge subquery parameters to the parameters of the main query.
+    if (!empty($subqueryparams)) {
+        $params = array_merge($params, $subqueryparams);
+    }
+
+    // Sub-query that fetches the list of unique events that were filtered based on priority.
+    $subquery = "SELECT ev.modulename,
+                        ev.instance,
+                        ev.eventtype,
+                        MAX(ev.priority) as priority
+                   FROM {event} ev
+                  $subquerywhere
+               GROUP BY ev.modulename, ev.instance, ev.eventtype";
+
+    // Build the main query.
     $sql = "SELECT e.*
               FROM {event} e
-         LEFT JOIN {modules} m ON e.modulename = m.name
-                -- Non visible modules will have a value of 0.
+        INNER JOIN ($subquery) fe
+                ON e.modulename = fe.modulename
+                   AND e.instance = fe.instance
+                   AND e.eventtype = fe.eventtype
+                   AND (e.priority = fe.priority OR (e.priority IS NULL AND fe.priority IS NULL))
+         LEFT JOIN {modules} m
+                ON e.modulename = m.name
              WHERE (m.visible = 1 OR m.visible IS NULL) AND $whereclause
           ORDER BY e.timestart";
     $events = $DB->get_records_sql($sql, $params);
diff --git a/calendar/tests/externallib_test.php b/calendar/tests/externallib_test.php
index b25406863a9..9baf24918be 100644
--- a/calendar/tests/externallib_test.php
+++ b/calendar/tests/externallib_test.php
@@ -62,7 +62,7 @@ class core_calendar_externallib_testcase extends externallib_advanced_testcase {
      */
 
     public static function create_calendar_event($name, $userid = 0, $type = 'user', $repeats = 0, $timestart  = null, $prop = null) {
-        global $CFG, $DB, $USER, $SITE;
+        global $CFG, $DB, $SITE;
 
         require_once("$CFG->dirroot/calendar/lib.php");
         if (!empty($prop)) {
@@ -100,6 +100,26 @@ class core_calendar_externallib_testcase extends externallib_advanced_testcase {
         if (!isset($prop->courseid)) {
             $prop->courseid = $SITE->id;
         }
+
+        // Determine event priority.
+        if ($prop->courseid == 0 && isset($prop->groupid) && $prop->groupid == 0 && !empty($prop->userid)) {
+            // User override event.
+            $prop->priority = CALENDAR_EVENT_USER_OVERRIDE_PRIORITY;
+        } else if ($prop->courseid != $SITE->id && !empty($prop->groupid)) {
+            // Group override event.
+            $priorityparams = ['courseid' => $prop->courseid, 'groupid' => $prop->groupid];
+            // Group override event with the highest priority.
+            $groupevents = $DB->get_records('event', $priorityparams, 'priority DESC', 'id, priority', 0, 1);
+            $priority = 1;
+            if (!empty($groupevents)) {
+                $event = reset($groupevents);
+                if (!empty($event->priority)) {
+                    $priority = $event->priority + 1;
+                }
+            }
+            $prop->priority = $priority;
+        }
+
         $event = new calendar_event($prop);
         return $event->create($prop);
     }
@@ -266,6 +286,7 @@ class core_calendar_externallib_testcase extends externallib_advanced_testcase {
         global $DB, $USER;
 
         $this->resetAfterTest(true);
+        set_config('calendar_adminseesall', 1);
         $this->setAdminUser();
 
         // Create a few stuff to test with.
@@ -296,13 +317,19 @@ class core_calendar_externallib_testcase extends externallib_advanced_testcase {
 
         $record = new stdClass();
         $record->courseid = $course->id;
+        $record->groupid = 0;
         $record->description = array(
             'format' => FORMAT_HTML,
             'text' => 'Text with img <img src="@@PLUGINFILE@@/fakeimage.png">',
             'itemid' => $draftidfile
         );
         $courseevent = $this->create_calendar_event('course', $USER->id, 'course', 2, time(), $record);
-        $userevent = $this->create_calendar_event('user', $USER->id);
+
+        $record = new stdClass();
+        $record->courseid = 0;
+        $record->groupid = 0;
+        $userevent = $this->create_calendar_event('user', $USER->id, 'user', 0, time(), $record);
+
         $record = new stdClass();
         $record->courseid = $course->id;
         $record->groupid = $group->id;
@@ -335,6 +362,13 @@ class core_calendar_externallib_testcase extends externallib_advanced_testcase {
         $this->assertEquals(2, $withdescription);
 
         // Let's play around with caps.
+
+        // Create user event for the user $user.
+        $record = new stdClass();
+        $record->courseid = 0;
+        $record->groupid = 0;
+        $this->create_calendar_event('user', $user->id, 'user', 0, time(), $record);
+
         $this->setUser($user);
         $events = core_calendar_external::get_calendar_events($paramevents, $options);
         $events = external_api::clean_returnvalue(core_calendar_external::get_calendar_events_returns(), $events);
diff --git a/calendar/tests/lib_test.php b/calendar/tests/lib_test.php
index 1d74d01ff6f..3e3bb394824 100644
--- a/calendar/tests/lib_test.php
+++ b/calendar/tests/lib_test.php
@@ -119,35 +119,42 @@ class core_calendar_lib_testcase extends advanced_testcase {
     public function test_calendar_get_events_with_disabled_module() {
         global $DB;
 
-        $course = $this->getDataGenerator()->create_course();
-        $events = [[
-                        'name' => 'Start of assignment',
-                        'description' => '',
-                        'format' => 1,
-                        'courseid' => $course->id,
-                        'groupid' => 0,
-                        'userid' => 2,
-                        'modulename' => 'assign',
-                        'instance' => 1,
-                        'eventtype' => 'due',
-                        'timestart' => time(),
-                        'timeduration' => 86400,
-                        'visible' => 1
-                    ], [
-                        'name' => 'Start of lesson',
-                        'description' => '',
-                        'format' => 1,
-                        'courseid' => $course->id,
-                        'groupid' => 0,
-                        'userid' => 2,
-                        'modulename' => 'lesson',
-                        'instance' => 1,
-                        'eventtype' => 'end',
-                        'timestart' => time(),
-                        'timeduration' => 86400,
-                        'visible' => 1
-                    ]
-                ];
+        $generator = $this->getDataGenerator();
+        $course = $generator->create_course();
+        $student = $generator->create_user();
+        $generator->enrol_user($student->id, $course->id, 'student');
+        $this->setUser($student);
+
+        $events = [
+            [
+                'name' => 'Start of assignment',
+                'description' => '',
+                'format' => 1,
+                'courseid' => $course->id,
+                'groupid' => 0,
+                'userid' => 2,
+                'modulename' => 'assign',
+                'instance' => 1,
+                'eventtype' => 'due',
+                'timestart' => time(),
+                'timeduration' => 86400,
+                'visible' => 1
+            ], [
+
+                'name' => 'Start of lesson',
+                'description' => '',
+                'format' => 1,
+                'courseid' => $course->id,
+                'groupid' => 0,
+                'userid' => 2,
+                'modulename' => 'lesson',
+                'instance' => 1,
+                'eventtype' => 'end',
+                'timestart' => time(),
+                'timeduration' => 86400,
+                'visible' => 1
+            ]
+        ];
 
         foreach ($events as $event) {
             calendar_event::create($event, false);
@@ -171,4 +178,186 @@ class core_calendar_lib_testcase extends advanced_testcase {
         $event = reset($events);
         $this->assertEquals('assign', $event->modulename);
     }
+
+    /**
+     * Test for calendar_get_events() when there are user and group overrides.
+     */
+    public function test_calendar_get_events_with_overrides() {
+        global $DB;
+
+        $generator = $this->getDataGenerator();
+        $course = $generator->create_course();
+        $plugingenerator = $this->getDataGenerator()->get_plugin_generator('mod_assign');
+        if (!isset($params['course'])) {
+            $params['course'] = $course->id;
+        }
+        $instance = $plugingenerator->create_instance($params);
+
+        // Create users.
+        $useroverridestudent = $generator->create_user();
+        $group1student = $generator->create_user();
+        $group2student = $generator->create_user();
+        $group12student = $generator->create_user();
+        $nogroupstudent = $generator->create_user();
+
+        // Enrol users.
+        $generator->enrol_user($useroverridestudent->id, $course->id, 'student');
+        $generator->enrol_user($group1student->id, $course->id, 'student');
+        $generator->enrol_user($group2student->id, $course->id, 'student');
+        $generator->enrol_user($group12student->id, $course->id, 'student');
+        $generator->enrol_user($nogroupstudent->id, $course->id, 'student');
+
+        // Create groups.
+        $group1 = $generator->create_group(['courseid' => $course->id]);
+        $group2 = $generator->create_group(['courseid' => $course->id]);
+
+        // Add members to groups.
+        $generator->create_group_member(['groupid' => $group1->id, 'userid' => $group1student->id]);
+        $generator->create_group_member(['groupid' => $group2->id, 'userid' => $group2student->id]);
+        $generator->create_group_member(['groupid' => $group1->id, 'userid' => $group12student->id]);
+        $generator->create_group_member(['groupid' => $group2->id, 'userid' => $group12student->id]);
+
+        $now = time();
+        // Events with the same module name, instance and event type.
+        $events = [
+            [
+                'name' => 'Assignment 1 due date',
+                'description' => '',
+                'format' => 0,
+                'courseid' => $course->id,
+                'groupid' => 0,
+                'userid' => 2,
+                'modulename' => 'assign',
+                'instance' => $instance->id,
+                'eventtype' => 'due',
+                'timestart' => $now,
+                'timeduration' => 0,
+                'visible' => 1
+            ], [
+                'name' => 'Assignment 1 due date - User override',
+                'description' => '',
+                'format' => 1,
+                'courseid' => 0,
+                'groupid' => 0,
+                'userid' => $useroverridestudent->id,
+                'modulename' => 'assign',
+                'instance' => $instance->id,
+                'eventtype' => 'due',
+                'timestart' => $now + 86400,
+                'timeduration' => 0,
+                'visible' => 1,
+                'priority' => CALENDAR_EVENT_USER_OVERRIDE_PRIORITY
+            ], [
+                'name' => 'Assignment 1 due date - Group A override',
+                'description' => '',
+                'format' => 1,
+                'courseid' => $course->id,
+                'groupid' => $group1->id,
+                'userid' => 2,
+                'modulename' => 'assign',
+                'instance' => $instance->id,
+                'eventtype' => 'due',
+                'timestart' => $now + (2 * 86400),
+                'timeduration' => 0,
+                'visible' => 1,
+                'priority' => 1,
+            ], [
+                'name' => 'Assignment 1 due date - Group B override',
+                'description' => '',
+                'format' => 1,
+                'courseid' => $course->id,
+                'groupid' => $group2->id,
+                'userid' => 2,
+                'modulename' => 'assign',
+                'instance' => $instance->id,
+                'eventtype' => 'due',
+                'timestart' => $now + (3 * 86400),
+                'timeduration' => 0,
+                'visible' => 1,
+                'priority' => 2,
+            ],
+        ];
+
+        foreach ($events as $event) {
+            calendar_event::create($event, false);
+        }
+
+        $timestart = $now - 100;
+        $timeend = $now + (3 * 86400);
+
+        $groups = [$group1->id, $group2->id];
+
+        // Get user override events.
+        $this->setUser($useroverridestudent);
+        $events = calendar_get_events($timestart, $timeend, $useroverridestudent->id, $groups, $course->id);
+        $this->assertCount(1, $events);
+        $event = reset($events);
+        $this->assertEquals('Assignment 1 due date - User override', $event->name);
+
+        // Get event for user with override but with the timestart and timeend parameters only covering the original event.
+        $events = calendar_get_events($timestart, $now, $useroverridestudent->id, $groups, $course->id);
+        $this->assertCount(0, $events);
+
+        // Get events for user that does not belong to any group and has no user override events.
+        $this->setUser($nogroupstudent);
+        $events = calendar_get_events($timestart, $timeend, $nogroupstudent->id, $groups, $course->id);
+        $this->assertCount(1, $events);
+        $event = reset($events);
+        $this->assertEquals('Assignment 1 due date', $event->name);
+
+        // Get events for user that belongs to groups A and B and has no user override events.
+        $this->setUser($group12student);
+        $events = calendar_get_events($timestart, $timeend, $group12student->id, $groups, $course->id);
+        $this->assertCount(1, $events);
+        $event = reset($events);
+        $this->assertEquals('Assignment 1 due date - Group B override', $event->name);
+
+        // Get events for user that belongs to group A and has no user override events.
+        $this->setUser($group1student);
+        $events = calendar_get_events($timestart, $timeend, $group1student->id, $groups, $course->id);
+        $this->assertCount(1, $events);
+        $event = reset($events);
+        $this->assertEquals('Assignment 1 due date - Group A override', $event->name);
+
+        // Add repeating events.
+        $repeatingevents = [
+            [
+                'name' => 'Repeating site event',
+                'description' => '',
+                'format' => 1,
+                'courseid' => SITEID,
+                'groupid' => 0,
+                'userid' => 2,
+                'repeatid' => 1,
+                'modulename' => '0',
+                'instance' => 0,
+                'eventtype' => 'site',
+                'timestart' => $now + 86400,
+                'timeduration' => 0,
+                'visible' => 1,
+            ],
+            [
+                'name' => 'Repeating site event',
+                'description' => '',
+                'format' => 1,
+                'courseid' => SITEID,
+                'groupid' => 0,
+                'userid' => 2,
+                'repeatid' => 1,
+                'modulename' => '0',
+                'instance' => 0,
+                'eventtype' => 'site',
+                'timestart' => $now + (2 * 86400),
+                'timeduration' => 0,
+                'visible' => 1,
+            ],
+        ];
+        foreach ($repeatingevents as $event) {
+            calendar_event::create($event, false);
+        }
+
+        // Make sure repeating events are not filtered out.
+        $events = calendar_get_events($timestart, $timeend, true, true, true);
+        $this->assertCount(3, $events);
+    }
 }