When I see similar questions which I can't or don't feel like closing as duplicates, I usually post a comment under one of the question with a link to the other one. This makes question linked - I don't know all the implications, but the obvious one is that both questions get hyperlinks to each other in the Linked sidebar. This improves navigation without any ethical implications that voting to close might have.
Having said that, if your self-answer covers an older question better than its own answers, I advice you to vote to close and let the community decide.