If it cannot be marked as duplicate, it means that the system as of now implicitly does allow duplicated question to exist over the network, so I think it should be answered.
Plus in this case the question is tagged
ubuntu, but in general an answer given to a question in a particular network might not fit well if given to the same question in a different network, i.e. the same question might have slightly different answers over different networks.
So the best thing to do would be:
- Contact the user and ask him whether he's willing to post the same answer on AskUbuntu as well, so that he can get credit and reputation from it
If he's not or he's not signed up on AskUbuntu then, as for regular answers, I think the "The link might go down" and "Posting links in answers without an explanation does not make up a good answer" criteria should apply, so in that case you should:
- Copy it
- Credit the user
- Add a link to the original answer, so that the user can get credit and reputation from it on the original network