Show learned pminions/mounts/barding in item description

[Niwashi]

Haven't tried this yet for minions, but can't you right-click (square button on PS4) and choose search to see if you already have it before purchasing?

I'm pretty sure that only checks your inventory and retainers' inventory, not your lists of learned skills.

There's a warning message if you buy gear that you're too low-level to equip, but you can still buy it if you want to. I think getting a duplicate of a tradeable minion should work the same way. "Do you want this item even though you cannot use it? Yes/No."

And as for UNTRADEABLE ones, you shouldn't even have the option to get another once you already have it.