This notification comes up, only if you any previous active membership under the same Adobe ID.
I have checked you adobe account and there isn't active membership however.
I would advise you to Contact Customer Care to get this issue fixed.
Yes, my teammate also has the same problem.
I am team administrator and I assigned a new user. And when she clicked to an email that she received to accept invitation, it asked to create an account and after the that it was saying "If you'd like to join "teamname", you can discontinue your pre-existing individual membership by contacting Customer Support ". But she just created account!!!