[-] BO : Fixing unique key blocking in some cases

If you have 2 groups sharing stock, then you can have this kind of line in ps_stock_available :
id_product=7, id_shop=0, id_shop_group=1
id_product=7, id_shop=0, id_shop_group=2
The actuel unique key don't permit these lines ! Isn't it a big bug ?