mysql - how to avoid duplicating in php -

i new in terms of php. creating simple program similar login page. may please me coding, have mysql table named user has 3 column userid, email , password. primary userid , auto increment. how can have code is, column email should no duplication or no same email. have code avoiding empty fields don't know how duplication.. here sample code:

<?php            if(isset($_post['submit'])){             $dbhost = 'localhost';             $dbuser = 'root';             $conn = mysql_connect($dbhost, $dbuser);             mysql_select_db('dtr');             if(! $conn ){                 die('could not connect: ' . mysql_error());             }              if(! get_magic_quotes_gpc() ){                 $email = addslashes ($_post['email']);                 $password = addslashes ($_post['password']);             }             else{                 $email = $_post['email'];                 $password = $_post['password'];             }                    //validation             if($email == ''){                 echo "empty ang email" ?></br><?php ;                 return false;             }                if($password == ''){                 echo "kailangan may password ka\n" ?></br><?php ;                 return false;             } ---------------------->//select * table username=user             {            $sql = "insert user "."(email, password) "."values('$email','$password')";         $retval = mysql_query( $sql, $conn );             }             if(! $retval ){                 die('could not enter data: ' . mysql_error());             }                 echo "entered data successfully\n";                 mysql_close($conn);         }             else                 {             }     ?> 

help me plz..

you can try:

$sql = "select userid users email = ". $_post["email"]; if (mysql_num_rows(mysql_query($sql, $con)) >= 1) {     echo "that email provided seems used";     return; } 

and please thing using different db extension since mysql deprecated of php v. 5.5. give better security features binding , prepared statements.