Duplicate Values (1 Viewer)

[Mik3]

Hi,
I need a validation rule that checks all values in a field for duplicates, before writing anything to the table.
I already have the field set not to accept duplicates, however the form would try to write the value before checking it. If the value is a duplicate, the auto number set as primary key would still increment, without anything being written, and that is a problem.

Thanks.

 

[Mik3]

I found this thread and it worked fine.
However the auto number still increments by one every time a user tries to enter a duplicate field.
Is there a way of preventing this?