Actually, I don't believe there's a pop-up that you receive for buying multiples of an item. I'm not sure what you're referring to, Maro.
The shops are simply, well, shops! They just show what items are available at the present moment. All of the items have colour names in the title, so "Crew Cut Orange," "Crew Cut Ash," etc.
If you'd like to know what you already have, you can always check your inventory to see if you have it or not.
That's just how we currently have it set up.
Hope that helps!