[hvidgaard]

Not OP, but I am working with a CQRS system.

In CQRS eventual consistency does not mean that we have multiple servers such that we have 2 servers with 2 different answers. It means from a command is issued to an event is propagated to all read models, there is a delay.

You need to handle the race condition at some point or another. From a CQRS point of view, a user accepting an invitation is just an event like any other event. What happens based on that event must account for the possibility that multiple users have accepted, and it's a rather straight forward thing to solve with an ES. The "accept" event with the lowest sequence number is the first.

Having the read side handle it would probably mean that when you have a read model for accepted invitations, you ignore all but the first accepted of an "invitationId".