I believe there may be a documentation error.
" Preventing Duplicate Field Values"
NOT(IN([_THIS], SELECT(Customers[State], NOT(IN([CustomerId], LIST([_THISROW].[CustomerId]))))))
I could not get the above to work. In fact when I tried to hit SAVE, it looked like it was synching but did not actually save anything and the SAVE button remained blue. No errors or warning. This problem is mentioned in the community as sometimes occuring.
I managed to get the desired result by the below code…which is also shorter, so even better!