Adding check in while loop entry - php

Question

Here in this code I am accepting data as an array and inserting it in the database. As I am using while loop I am having multiple input fields with same name as you can see in the HTML code below. What I want to do is, I want to add a check that if two fields are same then echo error saying two fields cannot be the same else insert the data. I could do it if there were two different input fields for the data like if($field1 == $field2){ echo "Error!"; else insert data. But here the field is only one with is giving multiple fields in a while loop. How can I add this check?
HTML form code:
<form method="post" action="<?php echo $_SERVER['PHP_SELF']; ?>">
<?php while($pro = $priq->fetch()){ extract($pro); ?>
<div class="row tbpad">
<div class="col-sm-3 col-xs-6">
<p><i class="fa fa-caret-right"></i> <?php echo $bk_title; ?></p>
</div>
<div class="col-sm-1 col-xs-1">
<div class="form-group">
<input type="text" class="form-control" name="priority[]" value="<?php echo $pr_priority; ?>">
<input type="hidden" class="form-control" value="<?php echo $bk_id; ?>" name="bkid[]">
</div>
</div>
</div>
<?php } ?>
<input type="submit" class="btn btn-orange" value="Set Priority" name="priority">
</form>
PHP code:
if(isset($_POST['priority'])){
$priority = (!empty($_POST['priority']))?$_POST['priority']:null;
$bkid = (!empty($_POST['bkid']))?$_POST['bkid']:null;
$iNumSets = count($priority);
for($i = 0; $i < $iNumSets; $i++){
$str = "INSERT INTO priorities(pr_book, pr_by, pr_priority)
VALUES('$bkid[$i]', '$sess', '$priority[$i]')";
$res = $PDO->prepare($str);
$res->execute();
if($res){
echo '<script>alert("Priority added successfully!");window.location = "view-order.php";</script>';
}else{
echo '<script>alert("Server Error! Please try back later.");window.location = "add-order.php";</script>';
}
}
}

Answer 1

Since $priority is an array which you get from $_POST['priority'], you could do the following:
$num_original = count($priority);
$num_unique = count(array_unique($priority));
if ($num_original != $num_unique) {
echo "One or more item were idendical!";
}
Also make sure you use mysql_real_escape_string() or similar methods on your $_POST input before you pass it into the SQL query. Otherwise your code is vulnerable to SQL injections!