If you just leveled up and noticed some challenges that have already been validated, this is completely normal. These are actually old challenges that you've already finished and that you've been rewarded for in the past. These challenges remain available for anyone who hasn't had the chance to complete them yet.
If you've already completed and validated them in the past, you won't be rewarded for them a second time.
But don't worry! There are still many levels and challenges left to discover! 😏