Form resubmission issue PHP SQL - php

Question

If I insert a new record into the database via PHP and then click the submit button and then each time I refresh the page it inserts the recently added record into the database. How do I stop this from happening? I don't want to redirect to another form after the submit button too. The weird thing is, if I click the submit button twice and then refresh the form it doesn't insert duplicates into the database. Why is this? Please help :/ thanks
Here's the code:
<div class="insertDiv">
<form method="POST" action="contracts.php">
<?php
if(empty($_POST['ContractDate']) && empty($_POST['ComputerId2']) && empty($_POST['CustomerId']) && empty($_POST['ContractLevel']))
{
}
else
{
include("dbinfo.inc.php");
$comm=#mysql_connect(localhost,$username,$password);
$rs=#mysql_select_db($database) or die( "Unable to select database");
$contractDate=$_POST['ContractDate'];
$computerID=$_POST['ComputerId'];
$customerID=$_POST['CustomerId'];
$contractLevel=$_POST['ContractLevel'];
$sql="INSERT INTO contract VALUES ('','$contractDate','$computerID', '$customerID', '$contractLevel')";
$result=mysql_query($sql)or die("Insert Error: ".mysql_error());
mysql_close();
}
?>
<div class = "myButton">
Insert
</div>
<p></p>
Enter contract start date:&nbsp
<input type="date" name="ContractDate" size=30 class="input"><br><br>
Enter computerID:&nbsp
<input type="text" name="ComputerId" size=30 class="input"><br><br>
Enter customerID:&nbsp
<input type="text" name="CustomerId" size=30 class="input"><br><br>
Enter contract level:&nbsp
<input type="text" name="ContractLevel" size=30 class="input"><br><br>
<input type="reset" value="Reset" class="button">&nbsp&nbsp&nbsp&nbsp
<input type="submit" value="Submit" class="button">
</form>
</div>

Answer 1

check with some value from form which is already exists in table if no then insert the data into the table
or
1.generate a random string and store it in session,
2.then output it to your form as a hidden value,
3.check the submitted and store variable, if matches process your request,
4.go to 1.

Answer 2

Just make sure the form has only been submitted once, by using sessions.
This should work (I haven't been able to test it)
<?php
// you should start the session before sending any output to the browser
session_start();
if($_SESSION['prevent'] && $_SESSION['prevent'] == $_POST['prevent'])$valid=true;
$_SESSION['prevent']=sha1(mt_rand(0,9999999));
?>
<div class="insertDiv">
<form method="POST" action="contracts.php">
<?php
if(empty($_POST['ContractDate']) && empty($_POST['ComputerId2']) && empty($_POST['CustomerId']) && empty($_POST['ContractLevel']))
{
}
else
{
if($valid){
include("dbinfo.inc.php");
$comm=#mysql_connect(localhost,$username,$password);
$rs=#mysql_select_db($database) or die( "Unable to select database");
$contractDate=$_POST['ContractDate'];
$computerID=$_POST['ComputerId'];
$customerID=$_POST['CustomerId'];
$contractLevel=$_POST['ContractLevel'];
$sql="INSERT INTO contract VALUES ('','$contractDate','$computerID', '$customerID', '$contractLevel')";
$result=mysql_query($sql)or die("Insert Error: ".mysql_error());
mysql_close();
}
}
?>
<div class = "myButton">
Insert
</div>
<p></p>
Enter contract start date:&nbsp
<input type="date" name="ContractDate" size=30 class="input"><br><br>
Enter computerID:&nbsp
<input type="text" name="ComputerId" size=30 class="input"><br><br>
Enter customerID:&nbsp
<input type="text" name="CustomerId" size=30 class="input"><br><br>
Enter contract level:&nbsp
<input type="text" name="ContractLevel" size=30 class="input"><br><br>
<input type="reset" value="Reset" class="button">&nbsp&nbsp&nbsp&nbsp
<input type="hidden" value="<?php echo $_SESSION['prevent']; ?>">
<input type="submit" value="Submit" class="button">
</form>
</div>

Answer 3

Have a read of this article and it'll explain how to redirect so refreshing doesn't resubmit the form.
http://en.wikipedia.org/wiki/Post/Redirect/Get