I'm not sure if this question has been asked before. Basically, some questions have answers in other websites which are somehow related to this site. For example, there are several examples in here texample.net. Now if someone asks a question and it happens that the answer is in the aforementioned website, can we consider the question duplicate? An example for this scenario is in here where the solution is in this link . I'm asking this because external links can be removed or moved any times.
Questions can only be considered duplicates or other questions on the same network/site. In both instances one could answer the question with a link as reference (and therefore proper attribution), but include the main components of the linked answer as part of your answer. That way you preserve the answer on the page people read the question, and avoid having to click-through to another location. A secondary benefit is that the answer is preserved (and therefore still meaningful) even if the attribution link fails.
From the network's perspective, such questions are not duplicates since they're not closed as such. That is, they have a valid answer that is not duplicated on the specific site that the community can vote on. In some such instances, users who post such linked answers typically do so as a community wiki post (since such posts do not garner reputation for the answerer).