Happens more often than you might think www.thepoke.co.uk/2020/04/08/this-story-of-the-guy-who-lost-his-cat-and-ended-up-with-two-of-them-is-the-only-tale/
(, Sun 25 Apr 2021, 11:35, Reply)
Maybe they are, or look like, litter mates.
Incidentally, I lost a black cat a few years ago, last year two black cats turned up.
(, Sun 25 Apr 2021, 17:00, Reply)