If the cache topology changes while executing a putIfAbsent operation, the old primary owner will throw an OutdatedTopologyException, and the originator will retry on the new owner.
When retrying the PutKeyValueCommand on the new primary owner, we compare the current value with the command's new value. If they are equal, we assume that the initial command wrote the old value, and we return null.
However, the value might have been written by another putIfAbsent operation. So we could have two putIfAbsent(k, v) operations, both returning null.