doubt(user account activation)

Question

i have a doubt regarding email verification....after an user registers at the site an email goes to his inbox showing his userid,password and a link to activate his account....suppose if the user clicks the link 2nd time how the program detects his account has already been activated and one more thing when he logs into the site and if the account has not been activated how the program detects his account has not been activated
if anybody does have a sample code please post here or explain the concept