Unset post variables after form submission

My solution for this is doing a meta refresh if a post variable is set the $_POST do not follow it after the refresh.

<head>
<?php
  if (isset($_POST['Task'])){
  echo' <meta http-equiv="refresh" content="0; url=./ThisFile.php">';
  }
  ?>
</head>

If your form is going to be updating a database, then you can update or insert only if record does not exist. Simply do a select statement first. This will prevent duplicate records due to refresh.

If you are inserting the contents of this form to database, simply use the php/sql SELECT statement to check if "username" has already been submitted. If yes, throw an error. Like so:

//Database Values: servername, dbname, username and password;
//Also set ($_POST['username']) = $username or anything you like; 
$usererror = ""; //Will be filled later.
try { 
$conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
$conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

$stmt1 = $conn->prepare("SELECT * FROM yourdb WHERE username=:username");
$stmt1->bindParam(':email', $email);
$stmt1->execute();
$stmt1->fetch();

$querycheck = $stmt1->rowCount();
if ($querycheck >= 1) {
$usererror = "User already exists.";
}
}
catch(PDOException $e) {
// roll back the transaction if something failed
$conn->rollback();
echo "Critical Error. Contact admin immediately.";
error_log("Error: " . $e->getMessage(), 0); //logs connection errors instead of echoing the errors (for security reasons.)
}
$conn = null;

Then you can now insert only the form has no error:

If (empty($usererror)) {
//insert command 
}

Notice I used prepared statements, because on many articles, it is said to be the safest method to operate on Databases using php+sql. You can as well use the normal method if you don't really have security concerns, it'll save you more code lines too.

Also notice the $querycheck >= 1, it is very essential to ensure 1 insertion and never more than 1 even if the user refreshes 2,000 times, clicks back button and submit again etc. The mistake that tied me down and drove me mad for days was using $querycheck > 1 instead of $querycheck >= 1, that way, my forms kept submitting twice before throwing "Already Exists" error. Finally, I've been able to fix it. Hope this helps people that will visit this question later.

You can just reset the $_POST array after processing form data, for example like below code

<?php
if( isset($_POST['submit']))
{
//do some processing related to form;
//after done.....
$_POST = array(); //this declares post as empty array 
//which is global variable anyways.
}
?>

Or the second way is to use the hidden form thing....

<?php
if( isset($_POST['submit']))
{
//do some processing related to form;

//create a form with hidden field
?>
<form method="post" name="myform" action="actionpage">
<input type="hidden" name="somename" value="">
</form>
<?php
echo "<script> document.forms['myform'].submit();</script>";
}
?>

This will basically submits the form with 1 field with null value, I don't know if this is optimal but it fulfills need here.

Instead, you could use a captcha such as Recaptcha that doesn't allow post submission without doing the captcha. This is what I used and it works perfectly.