Hi every one, I know that maybe a lot of you had past for this same situation.
I have a table call INS and the column I am trying to secure not have duplicates is [COD], the situation is that I use a code that apply perfect when is a New record, the probles is that this code make that when I change something in a already existing record recognice like de value in [COD] is duplicated.
NOT(IN([_THIS],SELECT(INS[COD], [COD] = [_ThisRow].[COD])))
That is the code in the Valid_if, I think that the solution is that this column have the same function of a Primary Key column