It's because of the sequence of actions the game performs. The game remembers that you have regular witcher steel sword on Geralt's back, then you sell it and buy another sword. The problem is that game refresh info about swords in inventory slots after closing inventory menu, so for a moment game thinks you have 2 swords and creates 2 scabbards. Only after this game removes the sword from your inventory slot. I can't fix it, it's somewhere deep in compiled files I don't have access to. I'd need source files from CD Projekt but getting them is rather impossible.