I’d keep it, they aren’t going to pursue you legally over it because that would cost more than the trouble of it, especially with them being based in Malta most likely.
Didn’t this happen a while back with LeoVegas? They had a competition running or something and it glitched and ended up paying quite a few people £1000 followed by an email asking for it back only for them to send an email after that saying to keep it as a gesture of good will. I’m sure a member posted a story like that a few months back..