Quote Originally Posted by Fax_Celestis View Post
An item giving proficiency carries its own price. As for stacking bonuses? Insight bonuses are remarkably easy to acquire.
If you read the item description you'll discover that if you get the bonuses you don't get the proficiency (because you already have it). It wouldn't make sense to charge you extra for an ability you're never going to use.

It is not "remarkably easy" to acquire an insight bonus on attacks. Not a single core item provides it and other than true strike the only core spell that can do it is the 8th level moment of prescience. If you get one from an ability, feat, spell or item in a supplement it's usually to give you a bonus that will most likely stack with the bonuses you already have.