Apple ID Family Sharing between different apple ID's.

When I try to join my dad's apple ID family account with my new iPhone 11, it says @@firstName@@ is already part of your family. I have my own apple ID, so I don't know why it won't let me join. I definitely haven't joined the family as I have none of the benefits like apple music, which he has on his apple ID devices, and the iTunes, apple store purchases etc. Can someone help me with this issue?