Currently, MediaWiki will ignore conflicts between edits of the same user, causing previous edits to be overwritten, if there were no edits by other users since the edit's base revision (and the current edit is not a section edit). The relevant code is at https://phabricator.wikimedia.org/source/mediawiki/browse/master/includes/EditPage.php;56ade0650d492e01d2689dc918e7db3d56d18147$1997
This behavior seems undesirable: If I have the same page open in two tabs for some reason, and edit in both (possibly after hours or days), the second edit will undo the first edit, unless someone else has edited in the meantime. There seems to be no documented justification for this.
Originally, conflict detection would be disabled if the current user was the creator of the edit's base revision, no matter if other users edited. This behavior can be found the first revision of MediaWiki on record in git, Lee Daniel Crocker's "Initial revision" from 2003: https://phabricator.wikimedia.org/source/mediawiki/browse/master/includes/Article.php;d82c14fb4fbac288b42ca5918b0a72f33ecb1e69$419
Later, brion changed this so merging would work on section edits, but conflicts on whole page edits would still overwrite: https://phabricator.wikimedia.org/rMWc6e870e5c6f85a693e01c0c16ce027b4b7e45480
In 2008, Aaron added a check that made sure the current user was the only one to edit after the base revision: https://phabricator.wikimedia.org/rMW43129ccd4426f3d059b7851cc8acc02360070601
This was rather tricky to track down, since the method was moved from Article to EditPage and then later to Revison (by me).
But the big question is: WHY do we do that? It seems like we are going out of our way to do the wrong thing in an edge case. Am I missing some obvious benefit of this?