That's not something that should happen and doesn't in a typical FileMaker database. Users are and should be locked out of editing the same record but not different records. You'll need to take a closer look at the design of your system and exactly what is going on at the time a user gets this error message to figure out why this error message is appearing.
my guess is because they are still editing the field that all generate from the same pool of records.
I suggest that you describe that part of your database design in much more detail.