When page B gets deleted, page A gets moved to title B, and then page B gets deleted again, it would not be easy to separate both groups of deleted revisions of page B by undeletion, as it would if page B were recreated from scratch, unless page A was created after page B was deleted. Therefore, I think it would be easier if the checkboxes in Special:Undelete were grouped by ar_page_id. Also, to partially fix T39465, if a deleted revision from one page id had the same timestamp as one from another page id, Special:Undelete should only undelete the revision(s) from the selected page id(s), not other revisions with the same timestamp.