This is not cross-posting, unless the same person asks the question there and here. (It might also be considered cross-posting if the exact same text were posted both there and here. While technically not cross-posting, that would be bad.) Furthermore, the issue is not whether or not the question is on-topic there or here--it's on-topic in both places. So neither of the meta questions you've mentioned are likely to be of much help in deciding this one.
I'm not sure what would be accomplished by having it in both places. If someone posts the question here without having realized it was on Super User, that means there is a need here (at least that one person's need). Then it can be answered here. One way it could be answered here would be by quoting from and linking to the answer to the corresponding question on Super User.
If nobody's asked it here, that suggests that Ubuntu users who have had this problem have searched and found the answer (on Super User, or elsewhere). That suggests it doesn't need to be here.
Asking a question here with the intention of answering it yourself is generally fine--there is even a checkbox when you ask a question, that lets you compose the answer at the same time and submit them together. However, when you ask and answer a question here intentionally, it should add a useful new resource to the Internet. I doesn't necessarily need to add new information in the strictest sense--it's okay of the information is possible to get elsewhere. But it should carry some substantial benefit compared to other available Internet resources.
It sounds like you're just talking about copying content. Unless you think the question or answer will be deleted on Super User, there's probably no good reason to do that. You might want to save the web page from Super User in case something like that happens. Then (with proper attribution) it could be posted here or elsewhere (though you should of course first consider the reason it was deleted there in case that were to impact its suitability in other places).