I asked a question on the Unix site, didn't get an answer soon enough for my taste, and then asked my question again at the non-Stack Exchange Ask Fedora, and got an answer there. So, should I answer my own question by copying the answer given over there, while giving credit to the person who answered my question, and then mark my answer as belonging to the community wiki?
They use the same license we do, so as long as you attribute it it should be fine. It'd probably be best if you wrote your own answer now that you've gone through the steps of solving your issue, as opposed to just copy/pasting the answer from Ask Fedora
I asked a question on the Unix site, didn't get an answer soon enough for my taste, and then asked my question again at the non-Stack Exchange Ask Fedora, and got an answer there.
That's cross-posting -- whether or not you waited a decent amount of time, and in fact there are sites that only allow explicit cross-posting if you do wait -- and IMO good netiquette says you should indicate this at the time of the cross post. There are also sites that will close your question if you get caught cross-posting without admitting it explicitly. Cross-posting is fine if you do it legitimately (by being explicit) but sort of inconsiderate otherwise.
So for example if you ask a question here and somewhere else simultaneously, you should add a note to both posts saying this was "Cross-posted to:" and a link.
If you ask a question here and then the next day ask the same question somewhere else, you should add the note to both posts when you create the second one.
As for cross-posting the answers, IMO that's good -- as in, much better than not doing so -- but you should of course credit the source.