I also see this problem when signing in again. I am only using google OAuth(through plugin published by bubble.io), not using the built-in email/password login.
The error is raised after FB login attempt for already signed up FB user. User email was not used previously for registration with email. I need to use previously saved user data so delete user is not a good idea.
Same issue here with a created email/password user : adding a new oauth connection works for the first time. then We cannot link your account to this email because there is already an account for this email.
Anyone have filled a bug a report?
If your user was previously signed up with email, they have to be logged in with email and then do the OAuth once (only one time) so that Bubble links the two. After that, the user can log in with either email or OAuth. If the user is logged out, and had not previously done OAuth while logged in, then Bubble shows the message you get.
In my app, in the settings/profile page for logged in users, I have an add Google and add FB button for logged in users to also add these OAuth options.