[otherme123]

What we did in the past was contact the editor first and ask if they have any problem with it being pre-published. Some do (most of them in our experience), some don't. The paper always change between arXiv and the journal, so the source of truth is the journal.

> I have had a paper reviewed, then rejected, only to see the same idea elsewhere a while after

I've seen worse. A grant being rejected by the grant reviewer, only to be re-submitted slightly changed by a reviewer's friend (this is a small world, we know each other) the next year. The inners of science suck, most of us keep doing it because we are like monks: we love God (Science), but we hate the Church.