Having the same issue here.
Exactly the same issue.
I have set the SKU to not required, Not unique, and is_user_define to 1 thats why I was able to remove it from the attribute set.
Now, I have changed all item’s SKU’s to XXXXX. All items have the same SKU now.
If, I will set the SKU to be unique again that will fix the issue, I need to change all duplicate SKU’s of more than 3000 items. A SUICIDE mission.
Please help. how to make SKU not unique from the processing of adding products not only in the data base. In the data base, its already not unique but its not letting me because it’s still saying its unique on a certain file in the magento installation which I cant find.