Invitee cannot join my Apple Family because it says they're already part of it.

I am attempting to invite someone to my Apple family, which only has me and 2 others. When this other person accepts my invitation, they receive a message that says "You are already a member of this family."

They are not. Not online, not on device, not on anything that has access to this family or its allowed apps.

When they go to their apple account, they are not attached to any family. I sent this person an invite years ago, but they never accepted that one.