- Can't add companion to journal or collection
- Error when attempting to learn pet: "This Spell is Not Available to You"
- Purchased a pet in the auction house that gives the message "Can't add companion to your account."
In WoW Classic you can only learn one of each pet in game.
In modern WOW, you can add up to three of the same pet to the collection unless the pet information indicates that the pet is unique. Unique modern WOW pets can only be learned once and include shop pets, promotional pets, and some pets acquired in game.
To learn a battle pet, you must have at least one other pet of the same level or higher as the pet you are attempting to add to the collection. If you cage your only level 25 pet, you won't be able to add it or any other level 25 pet to your collection until you have leveled a different pet to 25.