Issue #2349081 by Liam Morland: Added Integrity constraint violation: 1062 Duplicate entry 'xxx' for key 'nid_serial': INSERT INTO {webform_submissions}.
Issue #2358317 by Liam Morland: Fixed Serial numbers can be skipped if pages have no components.
Issue #2351471 by DanChadwick: Fixed When a Webform is cloned, the serial number of the clone does not start at 1.
Issue #2213945 by torotil: Fixed unique validation terribly slow on large databases (due to missing index).