We can see that TESO has released many new assistants in recent years. They all look great, but for players who already have an assistant with the same features, most wouldn't be willing to pay the same price for the same assistant with a different look. 5,000 crowns is not a trivial amount of money.
It would be a good measure to allow players who already have the same feature assistant to lower its price when they buy the same assistant again. This would help players who like to have more variety in their assistants to complete their collection, and for zos more players would be willing to pay for it.