Currently it seems it is not so clear what happens if person B posts a challenge that is a duplicate of another challenge person A had already sandboxed.
While any user contributions on SE are licensed under CC BY-SA can legally be copied and altered if attribution is provided, I still think it is impolite and disrespectful to "steal" (i.e. post without consent) something that someone else has already put effort in, even if you came up with the same idea yourself.
We do have a policy on using abandoned sandboxed questions, where consent is assumed if there is no objection within two weeks.
This leaves us with the question: What do we do with challenges that are dupes of sandboxed posts and were posted without the consent of the author of the latter?
Should they be closed? Can they stay open? Is there another alternative?