You have this many to many relationship:
I have a demo file built around this relationship
That explores this very issue with three different ways to prevent assigning a given contact to the same event more than once:
It explores using Field Validation, A diminishing value list (what you describe here) and a check box format as ways to keep this data entry error from happening.
If you are using FileMaker 12, open the demo file from the File menu and FileMaker will produce a new copy converted to the new FileMaker 12 format.