diff --git a/phpBB/includes/db/db_tools.php b/phpBB/includes/db/db_tools.php
index a793a2f313..f72498af9b 100644
--- a/phpBB/includes/db/db_tools.php
+++ b/phpBB/includes/db/db_tools.php
@@ -785,7 +785,7 @@ class phpbb_db_tools
 			{
 				foreach ($index_array as $index_name => $column)
 				{
-					if ($this->sql_index_exists($table, $index_name))
+					if ($this->sql_unique_index_exists($table, $index_name))
 					{
 						continue;
 					}
@@ -1121,7 +1121,7 @@ class phpbb_db_tools
 	}
 
 	/**
-	* Check if a specified index exists in table
+	* Check if a specified index exists in table. Does not return PRIMARY KEY and UNIQUE indexes.
 	*
 	* @param string	$table_name		Table to check the index at
 	* @param string	$index_name		The index name to check
@@ -1156,7 +1156,7 @@ class phpbb_db_tools
 			case 'firebird':
 				$sql = "SELECT LOWER(RDB\$INDEX_NAME) as index_name
 					FROM RDB\$INDICES
-					WHERE RDB\$RELATION_NAME = " . strtoupper($table_name) . "
+					WHERE RDB\$RELATION_NAME = '" . strtoupper($table_name) . "'
 						AND RDB\$UNIQUE_FLAG IS NULL
 						AND RDB\$FOREIGN_KEY IS NULL";
 				$col = 'index_name';
@@ -1183,12 +1183,14 @@ class phpbb_db_tools
 			case 'oracle':
 				$sql = "SELECT index_name
 					FROM user_indexes
-					WHERE table_name = '" . $table_name . "'
-						AND generated = 'N'";
+					WHERE table_name = '" . strtoupper($table_name) . "'
+						AND generated = 'N'
+						AND uniqueness = 'NONUNIQUE'";
+				$col = 'index_name';
 			break;
 
 			case 'sqlite':
-				$sql = "PRAGMA index_info('" . $table_name . "');";
+				$sql = "PRAGMA index_list('" . $table_name . "');";
 				$col = 'name';
 			break;
 		}
@@ -1201,6 +1203,135 @@ class phpbb_db_tools
 				continue;
 			}
 
+			// These DBMS prefix index name with the table name
+			switch ($this->sql_layer)
+			{
+				case 'firebird':
+				case 'oracle':
+				case 'postgres':
+				case 'sqlite':
+					$row[$col] = substr($row[$col], strlen($table_name) + 1);
+				break;
+			}
+
+			if (strtolower($row[$col]) == strtolower($index_name))
+			{
+				$this->db->sql_freeresult($result);
+				return true;
+			}
+		}
+		$this->db->sql_freeresult($result);
+
+		return false;
+	}
+
+	/**
+	* Check if a specified index exists in table. Does not return PRIMARY KEY and UNIQUE indexes.
+	*
+	* @param string	$table_name		Table to check the index at
+	* @param string	$index_name		The index name to check
+	*
+	* @return bool True if index exists, else false
+	*/
+	function sql_unique_index_exists($table_name, $index_name)
+	{
+		if ($this->sql_layer == 'mssql')
+		{
+			$sql = "EXEC sp_statistics '$table_name'";
+			$result = $this->db->sql_query($sql);
+
+			while ($row = $this->db->sql_fetchrow($result))
+			{
+				// Usually NON_UNIQUE is the column we want to check, but we allow for both
+				if ($row['TYPE'] == 3)
+				{
+					if (strtolower($row['INDEX_NAME']) == strtolower($index_name))
+					{
+						$this->db->sql_freeresult($result);
+						return true;
+					}
+				}
+			}
+			$this->db->sql_freeresult($result);
+			return false;
+		}
+
+		switch ($this->sql_layer)
+		{
+			case 'firebird':
+				$sql = "SELECT LOWER(RDB\$INDEX_NAME) as index_name
+					FROM RDB\$INDICES
+					WHERE RDB\$RELATION_NAME = '" . strtoupper($table_name) . "'
+						AND RDB\$UNIQUE_FLAG IS NOT NULL
+						AND RDB\$FOREIGN_KEY IS NULL";
+				$col = 'index_name';
+			break;
+
+			case 'postgres':
+				$sql = "SELECT ic.relname as index_name, i.indisunique
+					FROM pg_class bc, pg_class ic, pg_index i
+					WHERE (bc.oid = i.indrelid)
+						AND (ic.oid = i.indexrelid)
+						AND (bc.relname = '" . $table_name . "')
+						AND (i.indisprimary != 't')";
+				$col = 'index_name';
+			break;
+
+			case 'mysql_40':
+			case 'mysql_41':
+				$sql = 'SHOW KEYS
+					FROM ' . $table_name;
+				$col = 'Key_name';
+			break;
+
+			case 'oracle':
+				$sql = "SELECT index_name, table_owner
+					FROM user_indexes
+					WHERE table_name = '" . strtoupper($table_name) . "'
+						AND generated = 'N'
+						AND uniqueness = 'UNIQUE'
+						AND index_name LIKE 'U_%'";
+				$col = 'index_name';
+			break;
+
+			case 'sqlite':
+				$sql = "PRAGMA index_list('" . $table_name . "') WHERE unique = 1;";
+				$col = 'name';
+			break;
+		}
+
+		$result = $this->db->sql_query($sql);
+		while ($row = $this->db->sql_fetchrow($result))
+		{
+			if (($this->sql_layer == 'mysql_40' || $this->sql_layer == 'mysql_41') && ($row['Non_unique'] || $row[$col] == 'PRIMARY'))
+			{
+				continue;
+			}
+
+			if ($this->sql_layer == 'sqlite' && !$row['unique'])
+			{
+				continue;
+			}
+
+			if ($this->sql_layer == 'postgres' && $row['indisunique'] != 't')
+			{
+				continue;
+			}
+
+			// These DBMS prefix index name with the table name
+			switch ($this->sql_layer)
+			{
+				case 'oracle':
+					$row[$col] = substr($row[$col], strlen('U_' . $row['table_owner']) + 1);
+				break;
+
+				case 'firebird':
+				case 'postgres':
+				case 'sqlite':
+					$row[$col] = substr($row[$col], strlen($table_name) + 1);
+				break;
+			}
+
 			if (strtolower($row[$col]) == strtolower($index_name))
 			{
 				$this->db->sql_freeresult($result);