Google O-auth and sign up with same email

# Issue

Currently it is possible to signup with an email id and use same email id to create another user with google O-auth.

# Requirement

Need to check in database for email id when signing up with O-auth and prevent signup if same email id is used previously to register an account.

No files or tests are named. Start by locating the Google OAuth signup entry point and the user database/model code, then trace how email-based accounts are stored. Done means an OAuth signup using an existing email is rejected and does not create a second user.

Written by the indexing model from the issue text.