RE: Related Tab showing Duplicate Entity
For anyone else facing the same issue. Simply open the form you/your organisation are using in the chosen entity. In our case, it was the account entity - Open the form, on the left hand side, you will see a list of the common entities. Look for the duplicate entry, click on it and then click on Remove on the top ribbon. Save the form and then publish the changes.
You will need to refresh / clear your browser cache or log out/log back into your environment for the changes to take place.
This should resolve the issue.
Hope this helps!