If a question appears already on a Stack Exchange website, should it be marked as a duplicate question?
I think not (and that seems to be the practice here).
Should a reviewer take any specific action when such a question is reviewed other than giving links to related questions as a comment?
PS: To clarify, I am interested in questions that are on-topic on multiple Stack Exchange sites. The related question What to do with cross-site duplicates? seems to focus on questions that are posted in one site but belongs to another.
To further clarify, my interest is in questions posted by another user on another website unaware of the first question (can happen when the tittle of the first question is not very specific).
Example: How can I understand the Coppersmith–Winograd algorithm? is a cross-site duplicate of Fast Matrix Multiplication.