I've had variations of this happening with all kinds of submits for decades and I've never understood why exactly.
I think the most common case is a double post in the forum. I also see new accounts sometimes double posting in the new account log thing once in a while (though only 1 account got created -- I think the 2nd attempt would generate an error for the submitter).
I used to think it may have been an errant double click (may not even be something the submitter notices) on the submit that somehow sends the page twice. Or, something like that. e.g.
https://www.bram.us/2020/11/04/preventing-double-f...I think when I've look at it, that's roughly what I see in the log... the same submit sent twice in the same second. So, the site just does both and you get double posts and such. Why/how that happens is a mystery, as far as I know.
When I search, I see people talking about adding JavaScript to prevent double posts... I hesitate to do that as it might just break the form in some cases (eg. make you re-enter it...grr).