Linking email with existing facebook user on Parse

Question

I've implemented the following functionality in my iOS app.

1. Sign up with an email as new PFUser with some details.
2. Sign up as new user using Facebook.

Now I want to link/merge Facebook with my existing account. For

1. I'm able to do it successfully.
2. I get an error that "this account is already linked to another user". What can be the reason.

What I think the reason behind this is

When the user signs up with facebook, a new pfuser is created on Parse which is linked to that user's Facebook already, so when I try to merge some other user it gives me an error. And also parse doesn't allow to merge two different pfusers.

I would really appreciate if someone can explain me.