My problem is kind of like a "no duplicate entries" option but it needs to be a bit more complex and I am hoping someone can help me out.

I have a form called Operation and there are two look-up fields; 1 is called Customers the other is called Drawing no. The user selects a Customer and then a drawing number [which is filtered by the Customer selected] then the user enters in the operation number which is a number valve.

My question is how do I make sure that the user can't duplicate an operation number that is related to a drawing number?

For example Drawing no: P1234 can only have an operation entry of 10; 20; 30. and the same again for a different drawing number. But they wouldn't be able to enter a operation number if it already exists with the same drawing number.

I hope that makes some sense?

Many thanks