“This phone number already exists on your account” error in Facebook

I was facing the same ” duplicate phone number” issue as well. I solved it by clicking “Forgot Password”, then search for my account, and clicked “I don’t have access to these” and I provided a new email address and new password. They sent a verification email to my new email address, I clicked into the Facebook verification link in my email to verify my account.

After 24 hours, I can log in using my new email address and new password. After that, I chose the new email address as my username to log in in future. Other email address (including old email address and the phone number) were removed.

This is how I solved my problem. It really works. I tried to look for Facebook help centre but there is no reply from them and they never publish this solution in their site. Do help to spread this and help others too.