How to disallow a user to with a same email access another mode of authentication

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Hello,

You can use the Management API within Auth0 Pre-Registration Action to check if the user with the same email already exists in another connection:

Here’s the sample Pre-registration Action you can create:

exports.onExecutePreUserRegistration = async (event, api) => {
  const ManagementClient = require('auth0').ManagementClient;
  const management = new ManagementClient({
      domain: event.secrets.domain,
      clientId: event.secrets.clientId,
      clientSecret: event.secrets.clientSecret,
  });
  management.getUsers({q: `email:"${event.user.email}"` },function (err, users) {
    if (users.length >= 1) {
      api.access.deny("The user with this email already exists");
    }
  });
}

I recommend installing the Real-Time Log Extension to debug Auth0 Extensibility such as Actions, Hooks and Rules.

I hope this helps!