This happened in Codeforces Round #709 (Div.2)

There were some technical difficulties during the round and the main site was temporarily down. I had a "pretests passed" submission on question 1 at 0:03 mark, but the UI was not updating correctly so I re-submitted the same solution at 1:44 mark. In the end I was awarded 242 points for that problem and only my last submission counted.

https://codeforces.me/contest/1484/standings/participant/111374935#p111374935