Not just on the same network. We only mark questions as duplicates if they are on the same site, e.g. a question can exist on both SU and SO wihtout being marked as duplicates. See: What to do with cross-site duplicates?
You can comment with a link to that other site, if you want to. If you wish to provide an answer, please make sure the answer is more than purely a link - and don't just copy entire answers from other sites, if possible. If you do, make sure to credit the source in accordance to their content licence (SE uses CC-BY-SA).
Generally, even if an answer can be found by searching we would prefer to also add the answer on SE, in line with our goals of being a single source of knowledge.