How to prevent repeated submission of forms in php_PHP tutorial

/*
* How to prevent repeated submission of forms in php
*/
session_start();
if (empty($_SESSION['ip'])) {//The first write operation, determine whether the IP address is recorded, so as to know whether to Write to the database
$_SESSION['ip'] = $_SERVER['REMOTE_ADDR']; //The first write will pave the way for subsequent refresh or retreat judgments
//..... ......//Write to database operation
} else {//There has been an operation after the first write, so it will no longer be written to the database
echo 'Please do not refresh and back off again' ; //Write some already written tips or other things
}
?>