Can I repost a challenge that I posted elsewhere here?

Yes! You can post it.

As you are owner of that post. So, you can do as you want with the post. You can read about license in the question also.

This post didn't use Codidact's import script; it was manually copied here. If the person is the same user as on the other site, that's kosher -- you always own your own work and can repost it anywhere else without any licensing considerations. While not definitive, a quick peek at the user profile on the SE question tells me that this user isn't participating on SE now and thus might have more reason to come here.

If the user who posted it here is not the same person as the author of the post there, then this would be a copyright violation, not a license violation. We're not equipped to do copyright investigations; if someone claiming to be the SE user comes to us and says "I'm not the one who reposted on your site and I object", then we'll investigate.[1] The simplest assumption is that it's the same person, and we can operate on that assumption until we have reason to doubt it.

The special icon and attribution for imports are for when we import somebody else's content. We're allowed to do that so long as we follow the attribution requirements of the license. The icon isn't required but is something we added to help people distinguish these questions if they want to. ~from the answer