Usually that doesn't happen because their replies don't conflict within the time interval of a round trip between the two of them, so one can see the other's action first, and decide to agree.
But if it does happen, either they get a new popup, eventually, saying that their conflict-resolution decisions also conflicted and what would they like to do, or the system decays to a CRDT-style pre-determined resolution rule for the second-order conflict -- with similar problems to the original CRDT resolution rule we tried to avoid. Things like "give priority to the person who chose to keep information instead of deleting", "give priority to the person with the biggest random number", "merge them by keeping information along with a note in the document about the attempt to delete the column", or "delete the column and put the edited cell in a note". But this time, only when the users conflict after the first popup, so it doesn't occur as often.
I think you're describing a metastability effect which occurs in all distributed systems that don't use pre-determined resolution rules like pure CRDTs. It happens in network protocols, distributed transactions, and consensus protocols in general. If the process you use to resolve depends on input from multiple parties who don't have a way to communicate and could choose conflicting values, there's a non-zero probability of needing another round or to escalate upwards to a higher-level system to resolve.
In many technical systems, provided there's a way to communicate it's not difficult to make the probability of repeated rounds or escalations tend arbitrarily close to zero (but not actually zero). If there's a possibility of network partition, though, the conflict can come back later when communication returns; there's no way to completely make it go away.
But if it does happen, either they get a new popup, eventually, saying that their conflict-resolution decisions also conflicted and what would they like to do, or the system decays to a CRDT-style pre-determined resolution rule for the second-order conflict -- with similar problems to the original CRDT resolution rule we tried to avoid. Things like "give priority to the person who chose to keep information instead of deleting", "give priority to the person with the biggest random number", "merge them by keeping information along with a note in the document about the attempt to delete the column", or "delete the column and put the edited cell in a note". But this time, only when the users conflict after the first popup, so it doesn't occur as often.
I think you're describing a metastability effect which occurs in all distributed systems that don't use pre-determined resolution rules like pure CRDTs. It happens in network protocols, distributed transactions, and consensus protocols in general. If the process you use to resolve depends on input from multiple parties who don't have a way to communicate and could choose conflicting values, there's a non-zero probability of needing another round or to escalate upwards to a higher-level system to resolve.
In many technical systems, provided there's a way to communicate it's not difficult to make the probability of repeated rounds or escalations tend arbitrarily close to zero (but not actually zero). If there's a possibility of network partition, though, the conflict can come back later when communication returns; there's no way to completely make it go away.