diff --git a/phpBB/phpbb/db/migration/data/v31x/migrations_deduplicate_entries.php b/phpBB/phpbb/db/migration/data/v31x/migrations_deduplicate_entries.php
new file mode 100644
index 0000000000..4e539cf36d
--- /dev/null
+++ b/phpBB/phpbb/db/migration/data/v31x/migrations_deduplicate_entries.php
@@ -0,0 +1,78 @@
+<?php
+
+/**
+ *
+ * This file is part of the phpBB Forum Software package.
+ *
+ * @copyright (c) phpBB Limited <https://www.phpbb.com>
+ * @license GNU General Public License, version 2 (GPL-2.0)
+ *
+ * For full copyright and license information, please see
+ * the docs/CREDITS.txt file.
+ *
+ */
+
+namespace phpbb\db\migration\data\v31x;
+
+class migrations_deduplicate_entries extends \phpbb\db\migration\migration
+{
+	static public function depends_on()
+	{
+		return array('\phpbb\db\migration\data\v31x\v3110');
+	}
+
+	public function update_data()
+	{
+		return array(
+			array('custom', array(array($this, 'deduplicate_entries'))),
+		);
+	}
+
+	public function deduplicate_entries()
+	{
+		$migration_state = array();
+		$duplicate_migrations = array();
+
+		$sql = "SELECT *
+			FROM " . $this->table_prefix . 'migrations';
+		$result = $this->db->sql_query($sql);
+
+		if (!$this->db->get_sql_error_triggered())
+		{
+			while ($migration = $this->db->sql_fetchrow($result))
+			{
+				$migration_state[$migration['migration_name']] = $migration;
+
+				$migration_state[$migration['migration_name']]['migration_depends_on'] = unserialize($migration['migration_depends_on']);
+				$migration_state[$migration['migration_name']]['migration_data_state'] = !empty($migration['migration_data_state']) ? unserialize($migration['migration_data_state']) : '';
+			}
+		}
+
+		$this->db->sql_freeresult($result);
+
+		foreach ($migration_state as $name => $migration)
+		{
+			$prepended_name = preg_replace('#^(?!\\\)#', '\\\$0', $name);
+			$prefixless_name = preg_replace('#(^\\\)([^\\\].+)#', '$2', $name);
+
+			if ($prepended_name !== $name && isset($migration_state[$prepended_name]) && $migration_state[$prepended_name] === $migration_state[$name])
+			{
+				$duplicate_migrations[] = $name;
+				unset($migration_state[$prepended_name]);
+			}
+			else if ($prefixless_name !== $name && isset($migration_state[$prefixless_name]) && $migration_state[$prefixless_name] === $migration_state[$name])
+			{
+				$duplicate_migrations[] = $name;
+				unset($migration_state[$prefixless_name]);
+			}
+		}
+
+		if (count($duplicate_migrations))
+		{
+			$sql = 'DELETE *
+				FROM ' . $this->table_prefix . 'migrations
+				WHERE '  . $this->db->sql_in_set('migration_name', $duplicate_migrations);
+			$this->db->sql_query($sql);
+		}
+	}
+}
diff --git a/phpBB/phpbb/db/migrator.php b/phpBB/phpbb/db/migrator.php
index d84635f33d..30eafcc470 100644
--- a/phpBB/phpbb/db/migrator.php
+++ b/phpBB/phpbb/db/migrator.php
@@ -212,12 +212,12 @@ class migrator
 		// Try falling back to a valid migration name with or without leading backslash
 		if (!isset($this->migration_state[$name]))
 		{
-			$appended_name = preg_replace('#^(?!\\\)#', '\\\$0', $name);
+			$prepended_name = preg_replace('#^(?!\\\)#', '\\\$0', $name);
 			$prefixless_name = preg_replace('#(^\\\)([^\\\].+)#', '$2', $name);
 
-			if (isset($this->migration_state[$appended_name]))
+			if (isset($this->migration_state[$prepended_name]))
 			{
-				$name = $appended_name;
+				$name = $prepended_name;
 			}
 			else if (isset($this->migration_state[$prefixless_name]))
 			{