In the event you receive a mount, pet, costume or personality that you already own from your Crown Crate, you'll be awarded a new resource - Crown Gems - in its place. You will also have the option to convert several items

It's important to note that Crown Crates are a form of microtransaction in ESO, and their contents are cosmetic or convenience-based. They do not directly impact gameplay or provide any significant advantages in terms of character power.