I noticed that one can add product with existing code and change a product code to a not unique one.
Log into the application as administrator.
Go to Administration > Products.
Click the "Edit" button next to any product.
Change the product's code to a not unique one (e.g. “C100”).
Try to update the product.
The Product is updated.
Click the “+Add Product” button.
Use a not unique product's code but with different casing (e.g. "c100").
Fill in all required fields and click the “Add Product” button.
The product is added.
It shouldn't be possible to change the product's code to a not unique one, even if the different casing is used;
It shouldn’t be possible to add the product with non-unique code;
The changes shouldn't be saved; instead, an error message should be returned.