PK XD pets are chosen at random, so you may receive a repeated pet. You can see the chances of receiving a certain type of pet inside the Pet Shop, next to the Pet Dex.
If you have reached the limit of pets you can have, you can say goodbye to one of your friends as follows:
Click on the pets menu on the cell phone,
then on the paw icon with the arrows,
and finally, select the option “pet farewell”
This decision is final and irreversible, so make sure you want to say goodbye to this pet before accepting!