stop inserting data in the database when refreshing the page

Header the user to a new page :

if (isset($_POST['submit'])) 
{
  $user= $_POST['username'];
  $email = $_POST['useremail'];
  $pass= $_POST['password']; 

  mysql_query("INSERT INTO table (username, useremail, email) VALUES(`$username','$useremail','$email')");

}
//best outside the if statement so user isn't stuck on a white blank page.
header("location: landing_page.php");
exit;

By doing this the user who refreshes will be refreshing landing_page.php which means it won't do the insert twice.

best advice: do a check to see if user exists first if so don't insert!

What is going on here is that when you refresh page, the form is submitted twice.

To prevent this, you can use sessions:

session_start();

if( $_SESSION['submit'] == $_POST['submit'] && 
     isset($_SESSION['submit'])){
    // user double submitted 
}
else {
    // user submitted once
    $_SESSION['submit'] = $_POST['submit'];        
}