Two or more content rows may currently duplicate each other (except for the content_id, of course). To make it easier to fix rev_parent_id for null revisions (eg. moves), we should create a maintenance script that deletes redundant duplicate rows from the content table and fixes slot_content_id for the corresponding rows in the slots table before we can create an extension in T223342 that will help fix rev_parent_id for many revisions. Only the smallest content_id in a set of duplicate content rows will be kept, and all the larger ones will be deleted.