The rule enforcing 1 NFT per hex wasn't implemented until after April 2023.
For miners after this, it could be because the miners became eligible to receive an NFT on the same day, thus they were both given one. (Rare case).
If thats not the case, then its because miner 1 received an NFT in another hex, then moved to a hex that already has an NFT miner in it (miner 2), then the NFT ring stays on miner 1 but the NFT itself shows as invalid (they are not in the NFT reward pool).