My comment on this phantom/imposter answer says:

This is the second time this has happened to me, a second, identical copy of my answer appears, and I seem to be open and active when I return to this tab. I see it open, and click save before noticing that it is a phantom/imposter post. In this case I was in the review cue in another tab at the same time, though I'm not sure if that's related.

I'm using the Chrome browser with several tabs open, but I am now absolutely sure that I never clicked the Add Another Answer button.

I thought I'd posted a question here the first time it happened to me, but I don't see it now. I'll see if I relocate the first double-answer (roughly a year ago).

question: How to make sure this doesn't happen again? It's not at all a major problem, I don't notice any adverse effects besides the poor aesthetics of having a second, deleted answer.

I haven't added the bug tag yet since I'd like get to the bottom of what's actually happening. The bug tag can sometimes attract a by-design labeler, who might clear up bugs by finding a plausible rationalization for the behavior, and thereby "solve" the issue prematurely.

edit: I remember that when I tried to save the phantom/imposter answer, I was presented with a Captcha and asked if I was a robot. I hadn't logged out during that brief span, so I'm not sure why. I believe that I dispensed with it by reloading the page. I can't remember exactly, but I didn't engage with it.