There are several things you can do in this circumstance.
Obviously, you can avoid posting the same question across multiple SE sites, if possible. But since you already have...
These are all mutually exclusive. Do only one:
If your question hasn't been upvoted, you can delete it yourself. (Though yours was upvoted...)
You can flag it for moderator attention, and ask that a moderator delete it using the "Other" field. Be sure to explain that it was cross-posted and answered on the other site.
You can copy the answer from your question on the other site to your question here. If you do this, be sure to attribute it properly by linking back to the answer on the other site and giving credit to the user who posted it there. This is fine since all user content on Stack Exchange is Creative Commons licensed. Or, you can ask the user to copy their own answer.
My advice is: If you think the answer might be useful on SO, to SO's readers, then go ahead and copy the answer. After all, the question's already here. If the question and answer would be totally useless on SO, then have your question deleted.