The one who posted the idea to the public first gets the priority; close the other as duplicate
Here, "the idea" includes all of the relevant aspects of a challenge which matters when deciding on the dupe-ness, i.e. the task (with details and test cases) and the winning criterion. We say a challenge X is a dupe of Y if a competitive answer of Y is very likely to be competitive in X.
While one can't reliably search for the 2k+ (and still growing) posts on the Sandbox, one can (and I believe one should) at least skim through the first page (50 questions) sorted by Active (which empirically covers around a week of activity) or simply text-search on it (say Ctrl+F). This applies especially when the challenge idea is directly based on a recent event (disclosure of a genome sequence in this case).
Edit: The title of this post applies only when both users have posted to main. Otherwise, we do have a policy on abandoned sandbox posts:
What should we do with abandoned proposals?
All content on Stack Exchange is licensed under CC BY-SA, so legally you are allowed to claim an
abandoned idea yourself to make it ready for main and post it.
However, doing so you should follow a few rules of courtesy. The
following process has been agreed on:
- If a challenge proposal was not edited or commented on for
a month, you can leave a comment that you would like
to take over the challenge, get it ready for main and post it.
- If the OP does not reply within two weeks,
telling you that they still intend to post the challenge themselves,
you are free to proceed with the challenge as you see fit.
- Don't repost, just edit the existing post.
You can use the following standard comment to express your interest
in a challenge:
This challenge proposal has been inactive for over a
month. I would like to take ownership of the challenge and make it
ready for posting. Please let me know within the next 14
days if you have any objections and would still like to
finish and post this challenge yourself.
While posting to main right away is not a recommended action even in this case, we may extend the 2-week implied consent rule, like "the challenge is OK to be kept open if the sandbox poster doesn't claim its ownership within 2 weeks".
That is, when the challenge in question is actually close enough to be considered a dupe.
If the challenges are not actually a dupe, handle them by their quality separately
In this particular case, the two challenges have the exact same task, but have different winning criterion (one is simple code-golf, the other is code golf with a special byte-counting rule). They are arguably close, but probably not enough so for a dupe. Assuming the sandboxed one has better quality (I don't say foolproof, but it's still better to sandbox than not):
- If the two challenges are not actually a dupe and of good quality, both can be kept open without problems.
- If the non-sandboxed one is of poor quality (e.g. unclear, which is usually the reason of being poor quality), I suggest to cast close votes with that reason, not as a dupe of the other.