stop inserting data in the database when refreshing the page

Question

I am looking for a way to stop inserting or sending data in the database when refreshing the page.

here is my code:

user_details_page.php

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Answer 1

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!