If you are using data tables then you can use the column changed event, if someone changes the dropdown in the data grid, it updates the data table.
You can do a for loop inside the event handler to check to see if the value is unique and change any values that are not unique.
Click the link for a list of datatable events and sample code:-
http://msdn.microsoft.com/en-us/library/9hx67se8(v=vs.100).aspx[
^]