Allow sso logging in with a existing user

[OmmyZhang]

I enabled sso login after there're already some users, who registered by interacting with a bot to act like sso. (In fact, I have just learned the special trick to using OAuth as OpenID from here )
So it would be great if there's an option in config to allow sso logging in with a existed user. Now it will show Error: mapping_error mxid {} is already taken .

[clokep]

I think this is a duplicate of #7633.