> I’ve never really thought about this - how does Outlook handle this?

Simple: It’s server based. These problems are trivial when the server is coordinating responses and the clients can only reserve a room if the server confirms it.

This is the problem that gets hand waved away with local first software that has multi user components: It doesn’t take long before two users do something locally that conflicts. Then what do you do? You have to either force a conflict resolution and get the users to resolve it, or you start doing things like discarding one of the changes so the other wins.