diff --git a/lib/db/install.xml b/lib/db/install.xml
index 51e9ac16601..aecf51e66eb 100644
--- a/lib/db/install.xml
+++ b/lib/db/install.xml
@@ -2230,6 +2230,7 @@
         <KEY NAME="primary" TYPE="primary" FIELDS="id"/>
         <KEY NAME="groupid" TYPE="foreign" FIELDS="groupid" REFTABLE="groups" REFFIELDS="id"/>
         <KEY NAME="userid" TYPE="foreign" FIELDS="userid" REFTABLE="user" REFFIELDS="id"/>
+        <KEY NAME="useridgroupid" TYPE="unique" FIELDS="userid, groupid" COMMENT="Unique key"/>
       </KEYS>
     </TABLE>
     <TABLE NAME="groupings_groups" COMMENT="Link a grouping to a group (note, groups can be in multiple groupings ONLY in a course). WAS: groups_groupings_groups">
@@ -3863,4 +3864,4 @@
       </INDEXES>
     </TABLE>
   </TABLES>
-</XMLDB>
\ No newline at end of file
+</XMLDB>
diff --git a/lib/db/upgrade.php b/lib/db/upgrade.php
index 4fa1f58ec07..0d34d582aa8 100644
--- a/lib/db/upgrade.php
+++ b/lib/db/upgrade.php
@@ -2373,5 +2373,32 @@ function xmldb_main_upgrade($oldversion) {
         upgrade_main_savepoint(true, 2018051704.03);
     }
 
+    if ($oldversion < 2018051704.04) {
+        // Remove duplicate entries from group memberships.
+        // Find records with multiple userid/groupid combinations and find the highest ID.
+        // Later we will remove all those entries.
+        $sql = "
+            SELECT MIN(id) as minid, userid, groupid
+            FROM {groups_members}
+            GROUP BY userid, groupid
+            HAVING COUNT(id) > 1";
+        if ($duplicatedrows = $DB->get_recordset_sql($sql)) {
+            foreach ($duplicatedrows as $row) {
+                $DB->delete_records_select('groups_members',
+                    'userid = :userid AND groupid = :groupid AND id <> :minid', (array)$row);
+            }
+        }
+        $duplicatedrows->close();
+
+        // Define key useridgroupid (unique) to be added to group_members.
+        $table = new xmldb_table('groups_members');
+        $key = new xmldb_key('useridgroupid', XMLDB_KEY_UNIQUE, array('userid', 'groupid'));
+        // Launch add key useridgroupid.
+        $dbman->add_key($table, $key);
+
+        // Main savepoint reached.
+        upgrade_main_savepoint(true, 2018051704.04);
+    }
+
     return true;
 }
diff --git a/version.php b/version.php
index 89c8802a5d7..ff4313d7530 100644
--- a/version.php
+++ b/version.php
@@ -29,7 +29,7 @@
 
 defined('MOODLE_INTERNAL') || die();
 
-$version  = 2018051704.03;              // 20180517      = branching date YYYYMMDD - do not modify!
+$version  = 2018051704.04;              // 20180517      = branching date YYYYMMDD - do not modify!
                                         //         RR    = release increments - 00 in DEV branches.
                                         //           .XX = incremental changes.