diff --git a/mod/forum/db/install.xml b/mod/forum/db/install.xml
old mode 100644
new mode 100755
index e1c33097e0d..cbd19068a57
--- a/mod/forum/db/install.xml
+++ b/mod/forum/db/install.xml
@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8" ?>
-<XMLDB PATH="mod/forum/db" VERSION="20160912" COMMENT="XMLDB file for Moodle mod/forum"
+<XMLDB PATH="mod/forum/db" VERSION="20171012" COMMENT="XMLDB file for Moodle mod/forum"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:noNamespaceSchemaLocation="../../../lib/xmldb/xmldb.xsd"
 >
@@ -118,6 +118,7 @@
       <KEYS>
         <KEY NAME="primary" TYPE="primary" FIELDS="id"/>
         <KEY NAME="forum" TYPE="foreign" FIELDS="forum" REFTABLE="forum" REFFIELDS="id"/>
+        <KEY NAME="useridforum" TYPE="unique" FIELDS="userid, forum" COMMENT="Unique key"/>
       </KEYS>
       <INDEXES>
         <INDEX NAME="userid" UNIQUE="false" FIELDS="userid"/>
diff --git a/mod/forum/db/upgrade.php b/mod/forum/db/upgrade.php
index 0ab1e6d8a39..7d398bbc839 100644
--- a/mod/forum/db/upgrade.php
+++ b/mod/forum/db/upgrade.php
@@ -84,5 +84,35 @@ function xmldb_forum_upgrade($oldversion) {
     // Automatically generated Moodle v3.3.0 release upgrade line.
     // Put any upgrade step following this.
 
+    if ($oldversion < 2017092200) {
+
+        // Remove duplicate entries from forum_subscriptions.
+        // Find records with multiple userid/forum combinations and find the highest ID.
+        // Later we will remove all those entries.
+        $sql = "
+            SELECT MIN(id) as minid, userid, forum
+            FROM {forum_subscriptions}
+            GROUP BY userid, forum
+            HAVING COUNT(id) > 1";
+
+        if ($duplicatedrows = $DB->get_recordset_sql($sql)) {
+            foreach ($duplicatedrows as $row) {
+                $DB->delete_records_select('forum_subscriptions',
+                    'userid = :userid AND forum = :forum AND id <> :minid', (array)$row);
+            }
+        }
+        $duplicatedrows->close();
+
+        // Define key useridforum (primary) to be added to forum_subscriptions.
+        $table = new xmldb_table('forum_subscriptions');
+        $key = new xmldb_key('useridforum', XMLDB_KEY_UNIQUE, array('userid', 'forum'));
+
+        // Launch add key useridforum.
+        $dbman->add_key($table, $key);
+
+        // Forum savepoint reached.
+        upgrade_mod_savepoint(true, 2017092200, 'forum');
+    }
+
     return true;
 }
diff --git a/mod/forum/version.php b/mod/forum/version.php
index 4cf8d418d69..3d140a92e9a 100644
--- a/mod/forum/version.php
+++ b/mod/forum/version.php
@@ -24,6 +24,6 @@
 
 defined('MOODLE_INTERNAL') || die();
 
-$plugin->version   = 2017051500;       // The current module version (Date: YYYYMMDDXX)
+$plugin->version   = 2017092200;       // The current module version (Date: YYYYMMDDXX)
 $plugin->requires  = 2017050500;       // Requires this Moodle version
 $plugin->component = 'mod_forum';      // Full name of the plugin (used for diagnostics)