Single Line - No duplicates

Single Line - No duplicates

HI I have a form with an inital lookup field. Lets call it Country. 

I then have a single field called City. What i am after is I cant choose USA from the top field and then enter Boston twice as that would be a duplicate. But i would want to be able to add UK from the lookup and type in Boston in the City.

Is this possible if so how do i do it? 

Any help massively appreciated. New to zoho, understand access but this is all new to me. 

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