You can trap for the error and display a warning message when such occurs. That involves setting up a TEXT field with an AUTO-Enter caldulation such as:
_kf_activity &"|" & _kf_participant
And then specifying a Unique Values validation rule.
I think though, that it's a better option to make it impossible to create the duplicate "join" record in the first place through the design of the user interface.
Here's a demo file that illustrates the method you've referred to but then also demonstrates two methods that prevent the creation of the duplicate join. One method is a diminishing value list where the list of available values only lists values not yet selected and another looks like a list of check boxes such that clicking an empty "check box" adds a new join record and clicking a "selected" check box deletes the join record:
Thanks! I think this the answer I'm looking for. However, I need more time to understand how it works and test it out. Cheers!
Feel free to ask any questions you have about how the demo file works. There are a number of subtle details that have to be set up just so before each of the methods demo'd work smoothly.
Thanks! Really appreciate the support!