is it legit to duplicate, self answer & redirect?
No. If you see a question you can answer, you should answer that question. You should not post a duplicate of that question, self-answer that, and then attempt to link the original question as a duplicate. This serves no purpose but to clutter the site and is essentially just a reputation grab.
That said, it's very understandable that somebody might do this accidentally, because they don't know the original question or questions exist. It's difficult or impossible to prove this scenario occurred with malice, so it's better to assume good faith and handle the scenario accordingly.
In general, given a set of candidate duplicate questions, the question with the best answers should be the target of the duplicate links. This ensures that users searching for help who end up on a duplicate find the solid answers they need as quickly as possible.
then answers X as a link to Y.
This is a "link only" answer and should be flagged for deletion as such. Just because the link is still to SE, and still to the same SE site, doesn't mean it won't suffer the same fate as any other link-only answer (the link may die for any number of reasons, making the answer useless).