Is it bad Stack Exchange etiquette to ask a duplicate of an Ask Ubuntu question on Super User and vice versa? (Assuming of course that said question is relevant on both sites.)
The last time we discussed this we didn't really have a problem (people getting problems solved is the priority) but most other stackexchange websites don't allow cross-posting.
The official line from upon high is to pick the best site and post there and only there. Write the best question you possibly can and if it doesn't pay off, ask for it to be migrated.