See this demo file that uses a many relationship between contacts and events to demonstrate a number of methods for linking contacts to events in that relationship. Three different methods for keeping the same contact from being selected more than once for the same event are demonstrated in this file. One is a dwindling value list where previously selected items are automatically omitted from the value list.
FileMaker 12 users can open this file from FileMaker's File menu to get a copy converted to the .fmp12 format.
Just what the doctor ordered! Many thanks!