diff --git a/tag/classes/area.php b/tag/classes/area.php
index 23a95edb60f00..7debad9a38178 100644
--- a/tag/classes/area.php
+++ b/tag/classes/area.php
@@ -412,14 +412,21 @@ public static function move_tags($component, $itemtype, $tagcollid) {
                 LEFT JOIN {tag} tt ON tt.name = t.name AND tt.tagcollid = :tagcollid2
                 WHERE ti.itemtype = :itemtype2 AND ti.component = :component2
                     AND tt.id IS NULL)";
-        $todelete = array();
+        $movedtags = array(); // Keep track of moved tags so we don't hit DB index violation.
         $result = $DB->get_records_sql($sql, $params);
         foreach ($result as $tag) {
             $originaltagid = $tag->id;
-            unset($tag->id);
-            $tag->tagcollid = $tagcollid;
-            $tag->id = $DB->insert_record('tag', $tag);
-            \core\event\tag_created::create_from_tag($tag);
+            if (array_key_exists($tag->name, $movedtags)) {
+                // Case of corrupted data when the same tag was in several collections.
+                $tag->id = $movedtags[$tag->name];
+            } else {
+                // Copy the tag into the new collection.
+                unset($tag->id);
+                $tag->tagcollid = $tagcollid;
+                $tag->id = $DB->insert_record('tag', $tag);
+                \core\event\tag_created::create_from_tag($tag);
+                $movedtags[$tag->name] = $tag->id;
+            }
             $DB->execute("UPDATE {tag_instance} SET tagid = ? WHERE tagid = ? AND itemtype = ? AND component = ?",
                     array($tag->id, $originaltagid, $itemtype, $component));
         }
diff --git a/tag/tests/taglib_test.php b/tag/tests/taglib_test.php
index 85add32029c6b..9eb2efbfdaaa0 100644
--- a/tag/tests/taglib_test.php
+++ b/tag/tests/taglib_test.php
@@ -813,6 +813,39 @@ public function test_move_tags_with_related() {
         $this->assertEquals('Tag2, Tag4', join(', ', $related21));
     }
 
+    public function test_move_tags_corrupted() {
+        global $DB;
+        list($collid1, $collid2, $user1, $user2, $blogpost) = $this->prepare_move_tags();
+        $collid3 = core_tag_collection::create(array('name' => 'weirdcoll'))->id;
+
+        // We already have Tag1 in coll1, now let's create it in coll3.
+        $extratag1 = $this->getDataGenerator()->create_tag(array('rawname' => 'Tag1',
+            'tagcollid' => $collid3, 'tagtype' => 'official'));
+
+        // Artificially add 'Tag1' from coll3 to user2.
+        $DB->insert_record('tag_instance', array('tagid' => $extratag1->id, 'itemtype' => 'user',
+            'component' => 'core', 'itemid' => $user2->id, 'ordering' => 3));
+
+        // Now we have corrupted data: both users are tagged with 'Tag1', however these are two tags in different collections.
+        $user1tags = array_values(core_tag_tag::get_item_tags('core', 'user', $user1->id));
+        $user2tags = array_values(core_tag_tag::get_item_tags('core', 'user', $user2->id));
+        $this->assertEquals('Tag1', $user1tags[0]->rawname);
+        $this->assertEquals('Tag1', $user2tags[2]->rawname);
+        $this->assertNotEquals($user1tags[0]->tagcollid, $user2tags[2]->tagcollid);
+
+        // Move user interests tag area into coll2.
+        $tagarea = $DB->get_record('tag_area', array('itemtype' => 'user', 'component' => 'core'));
+        core_tag_area::update($tagarea, array('tagcollid' => $collid2));
+
+        // Now all tags are correctly moved to the new collection and both tags 'Tag1' were merged.
+        $user1tags = array_values(core_tag_tag::get_item_tags('core', 'user', $user1->id));
+        $user2tags = array_values(core_tag_tag::get_item_tags('core', 'user', $user2->id));
+        $this->assertEquals('Tag1', $user1tags[0]->rawname);
+        $this->assertEquals('Tag1', $user2tags[2]->rawname);
+        $this->assertEquals($collid2, $user1tags[0]->tagcollid);
+        $this->assertEquals($collid2, $user2tags[2]->tagcollid);
+    }
+
     public function test_normalize() {
         $tagset = array('Cat', ' Dog  ', '<Mouse', '<>', 'mouse', 'Dog');