I don't think it's possible to submit the same code from different accounts without being detected by codeforces anti-cheat. I assume (or at least hope) Leetccode has a similar system and simply waits until the end of the contest to find cheaters, and that's why they are not blocked yet, but I don't know. In any case it's best when annoying cheaters are handled automatically, so there's probably a system for that in every major website.