Sites on the Stack Exchange network do tend to overlap in scope. It is not the case that a question asked on a more "generalist" site is automatically off-topic on that site once another site comes along that is more specific.
The more specific targeted sites, like AskUbuntu, Unix&Linux, etc. are designed to give users a community that shares a lot in common. This increases the likelihood of better quality, more direct answers. But that doesn't mean that those questions can't be asked on Super User anymore.
Generally, my guidelines would be:
If you know of a StackExchange community that specifically caters to the nature of the problem you're having, then ask your question there.
If you don't know of a site that is very "zeroed-in" on the issue you're having, but you can find a generalist site where it's on-topic, like Super User or Stack Overflow, then you can ask it there. If you get an answer that's useful, great! If not, after waiting a week or so for answers, you can request migration to a more specific site.
If another user asks a question on a "generalist" site that's clearly more likely to receive better answers on a "specialized" site, we should first attempt to answer it here on the generalist site without reference to the specialized site. If no one knows the answer after a while (a week or two), you can ask the OP if they would like to have it migrated to improve the likelihood of a better answer. Then the SU mods can work with another site's mods, vet the question (to verify that it's on-topic and of good quality), and migrate it if everything looks good.