Who ever writes last will overwrite the first entry. It is very unlikely that you will have issues as the users would literally have to change the same value within a refresh cycle of each other. Its acutally a selling point that multiple users can be in the same schedule.
Also, a message will appear if you try and schedule a project someone else has open. I would instruct them that if that happens they need to double check their work. They should be regularly hitting F5 to refresh their data.