title: Checking whether a user already exists in drupal
body: <p>When a user enters his login information and hits submit, i want to check if the user already exists or not.
  So, i have the following two questions
  1. Which hook is needed to be implemented , for the case when user hits the submit button on the login form. I need the username entered by the user.
  2. How to check if a user already exists in drupal or not programmatically ?</p>
  
  <p>Some sample code would be really appreciated.
  Please help.</p>
  
  <p>Thank You.</p>