An empty block can only claim the subsidy as the reward, which won’t equal the ~25 or ~50 BTC reward that all the potential duplicates claimed, so it wouldn’t be a duplicate in any event, even if they were given an exemption.
But an exemption isn’t necessary: empty blocks can include a witness commitment, it’ll just be constant (as the coinbase tx’s wtxid is replaced by uint256{0}, eg mempool - Bitcoin Explorer (a block with only the coinbase, the coinbase has the usual uint256{0} “witness reserved value” as its witness data, it has the witness commitment of aa21a9ed e2f61c3f71d1defd3fa999dfa36953755c690689799962b48bebd836974e8cf9 followed by the signet block signature. You can see other empty blocks have the same witness commitment, eg mempool - Bitcoin Explorer
EDIT to add: all the coinbases from the list in comment 4 have an nLocktime of 0 and an nVersion of 1, so requiring future blocks to have some different value for one or the other would also result in uniqueness.