Topic: Validating unique habtm?

I have a text box field that I'm using the populate my habtm relationship between widgets and gadgets.  I do not, however, want a gadget to be linked more than once to a widget.  I figure the best way to do this is using validation in the model, but I'm not sure how I would go about validation using habtm.  How could I achieve this?

Thanks in advance!