Duplicate field allowed only with checkbox = true

Hello, my question is in regards to a access 2007 database. I am trying to figure out the code to try to do 2 things in a form.



1. Prompt a error message when a duplicate number field comes up, (this is not a primary key but rather a stored number ID, number field) saying something like this is a duplicate ID band please select the checkbox duplicate first to add it.



2. Figure out the code to allow duplicates only when the checkbox is selected true.



Thanks guys I appreciate any starter code and hints.

Duplicate field allowed only with checkbox = true