This is handled by Redis, as such, the last write will win. As the primary key is tied to the user id, there will not really be a concept of Machine A vs Machine B as the end user can only be in one place at a time.
What happens to storage requests made while offline? Are they cached locally and propagated once online, or are they lost?
Judging by the title, I thought this tool actually just synced the existing localStorage instead of being its own storage. Since localStorage has onChange events it would be trivial to built, it would still be sync, it wouldn’t require any changes to the existing code, etc