I have seen same question but on different stackExchange.
What should I do now to eliminate same/repeated question?
When I notice this, I leave a comment under each version: "cross-posted on [the other site], with a link to it". This at least tells those who are considering writing an answer that their work may be redundant.
I also suggest to the author that cross-posting is discouraged and they should ask on one site at a time.
These comments usually have enough effect: one of copies gets either closed, or deleted, or migrated.