I have found some users are just copy-pasting same answers again and again instead of marking them duplicates.
Is this allowed on Ask Ubuntu?
If not, how to deal with such users?
Personally I don't upvote answers to questions which are clearly duplicates ("clearly" = I can find an almost exact duplicate in the first three pages using the search function or in the first page using Google), because that encourages answering duplicate questions instead of closing them, which is the Right Thing(TM).
That being said if I catch an user copy-pasting old answers of them to a duplicate question, I downvote and flag the copy-pasted answer (and vote to close the question).
If the questions are reasonably different, I don't have a problem with that at all; it answers the question, so what's the problem? I'm pretty sure it has happened to me as well at least once to have posted an answer that later has been deserveful to be posted as an answer to another question.
I am fine with it if the answer might be the same but the questions are not really duplicates of each other. Like two different problems but only one solution that works for both.
If also the questions are about the same, I'd rather go for the duplicate instead. However, it's not really bad in my opinion if one answer appears at two similar questions or to questions that are later marked as duplicates of each other. As long as this does not become a bad habit, I would not interact. When we can observe that behaviour very regularly though, I'd leave him a comment. Nothing more.