Ok, I watched myself and experimented a little and I found that it happens when I accidentally make a fast double click on the "publish" button, only in WikiEditor (not in VE).

Wow! It happens even when creating a page!
https://el.wikipedia.org/w/index.php?title=%CE%92%CE%BF%CE%AE%CE%B8%CE%B5%CE%B9%CE%B1:%CE%A0%CF%81%CF%8C%CF%87%CE%B5%CE%B9%CF%81%CE%BF/%CF%80%CF%81%CF%8C%CF%87%CE%B5%CE%B9%CF%81%CE%BF&action=history
Choosing the "previous version" (although there was none) resulted in two edits in page history (???)

Hey @geraki, thanks for the report and for creating this ticket.

Since you've been experimenting with this bug a bit already, could you please try to verify if this is a problem with the new edit conflict interface (Δυο στήλες στη σύγκρουση επεξεργασίας - currently a beta feature )? - Or do you also get an edit conflict screen using this method without the new interface?

another example/occurrence

https://www.mediawiki.org/w/index.php?title=Topic:V19gqc98l1f6vlil&topic_showPostId=v1iavivxu3t7p9pt#flow-post-v1iavivxu3t7p9pt

Still getting spurious edit conflict flagged even after beta feature switched off, and it's not a function of quickly double-clicking the "Publish" button.

In T225656#5406798, @David_Biddulph wrote:

Still getting spurious edit conflict flagged even after beta feature switched off,

Ok if this still happens even after the switched of beta feature, then it's clearly not a Two-Column-Edit-Conflict-Merge issues alone. Two-Column-Edit-Conflict-Merge just offers a new interface, the code that originally triggers edit conflicts lies somewhere else. Due to the fact, that the original post was about edit conflicts with yourself I will merge this into T222805: Edit conflicts for no reason with older revisions or yourself. - I hope this is fine for you @geraki

and it's not a function of quickly double-clicking the "Publish" button.

@David_Biddulph if you can give examples for edits that should not end up in edit conflicts but still do for you and also appear with edits from other users and your edits. Please create a new ticket. - That might be another problem then. - Thanks!

As an example, https://en.wikipedia.org/w/index.php?title=Roger_Burnley&type=revision&diff=910764771&oldid=910760450 was accepted but flagged up an edit conflict as if my edit hadn't been saved.