Thanks for your help...
I have a form with only two tables. The parent table stores Sessions with a Start Date and End Date. The child table stores data about a tower and there can be many towers. There should only be on tower entered per session, but there are 25 different towers.
I want the form to check if a tower has been entered during a session, and if it has, don't allow them to enter it again.
Session 1
Tower 29
Tower 40
Tower 42
Tower 6
Tower 2
Etc
Session 2
Tower 29
Tower 40
Tower 42
Tower 6
Tower 2
etc.