"Duplicate entry '' for key 2" but form is allowed duplicate entries.

"Duplicate entry '' for key 2" but form is allowed duplicate entries.

While entering data into my product form which is linked with look up fields to other forms, i am getting an error 
"Duplicate entry '' for key 2"

I have checked if the fields are set to reject duplicate entries which they are not and I am still getting this error. Originally they were set this way as this is what i want.  However i changed it thinking it would fix the problem, it has not. Also in looking at the fields, there are no duplicate entries.

I don't know but maybe this could be related to my last problem that was just fixed.

Please help.

App: Art Inventory Management 2.0
Form: Product
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