What I need is to avoid duplication of the document.

[Carlos2013]

I ask for your help:
I have an invoice that contains four types of numbers and letters, example:
ID Attorney Suc Nº Fact letter
20-12345678-9 0001 00000012- A
What I need is to avoid duplication of the document. This comparison must be done once the type of letter you enter.Thanks Carlos

[Rabbit]

Put a unique index on those columns.