"Not Duplicate" for Email Field is NOT WORKING

"Not Duplicate" for Email Field is NOT WORKING

We would check "not duplicate" under options in the edit option for the email field of our form. We tested it by adding a duplicate email and the duplicate was successfully saved. We went back to the edit field and the "not duplicate" box was not checked. We redid this several times. The "not duplicate" status does not save.
sheryl