You can set the field options to not allow users to override the error.
OnObectValidate, can be used to detect the error, use show custom dialog to tell the user of the error and then revert the records.
Set Error Capture [on]
IF [ Get ( LastError ) //duplicate record error ]
Show Custom dialog ["Error, duplicate value...." ]
Exit Script [False]
It is also possible to design the interface so that it is not possible to select the same pair of _Fk value in the join table more than once.
A diminishing value list can do this.
A kind of "check box" format can be set up where clicking a button once selects a value and clicking it again, deletes the join table record created by the first time the button was clicked and thus "clearing" the "check box".
"Adventures in FileMaking #1 - conditional value lists" contains a diminishing (dwindling) value list example.
"Adventures in FileMaking #2 - enhanced value selection" contains some examples of the "check box" format, but does not have an example where the script interacts with a join table (but has some hints about how to do this as a kind of "extra credit" assignment.)
Thanks for the help! will test the script and diminishing value!