Could we get something similar to 'Already in your Beanbag collection' etc. for the wardrobe? Or would that be in some way a problem?

Meh.

This might be helpful if adding items in one-by-one was the only option. But you can use quickstock and click the Wardrobe column to attempt to stuff everything in your Inventory that's wearable into your Wardrobe. If it's already in there, it stays in your Inventory and you get an error message on your quickstock results page.

I wouldn't mind this at all, there are several items I need multiples of, for wardrobe + TCs but I haven't wanted to add them to my WL multiple times (because that's a pain). It would be nice to be able to tell where I need the item(s) still without having to try putting it in my wardrobe first and getting an error