Allow only 1 POST of each dropdown value in PHP form - php

Question

I have a simple PHP form to insert "models" (in dorpdown) with a price.
The thing I want to do is that you can only post model 1 once and model 2 once etc..
So if you choose model 1 in the dropdown and post it and try it again that it doesn't allow you because you already posted model 1.
sorry for my bad English
<select name="model">
<?
$modellen[1]= "Model 1";
$modellen[2]= "Model 2";
$modellen[3]= "Model 3";
foreach ($modellen as $key => $value)
{
echo "<option value='".$key."''>".$value."</option>";
}
?>
</select>
if(isset($_POST['toevoegen']))
{
$prijs = Safesql($_POST['prijs']);
$moment = date("Y-m-d h:i:sa");
$kiesmodel = Safesql($_POST['model']);
if(!$mysqli->query("INSERT INTO prijzen (prijs, created, model) VALUES (".$prijs.",'".$moment."' , '".$kiesmodel."')")) {echo $mysqli->error;}
else{ echo "het toevoegen is gelukt";}
Laden(0);
}
//show records from database
if ($query = $mysqli->query("SELECT * FROM prijzen")) { echo $mysqli->error;}
if ($query->num_rows >= 1)
{
while($row = $query->fetch_assoc())
{
?><tr><td><form action="index.php" method="post">
<? echo "Model" . " " . $row['model']; ?><img src="delete.png">
<img src="edit.png"><?
echo " prijs:" . $row['prijs']. "<br>" ."";?><?
}
}

Answer 1

try add validation before insert,
if(isset($_POST['toevoegen']))
{
$prijs = Safesql($_POST['prijs']);
$moment = date("Y-m-d h:i:sa");
$kiesmodel = Safesql($_POST['model']);
// validation
$sql = "SELECT * FROM prijzen WHERE model = '$kiesmodel'";
$query = $mysqli->query($sql);
$row = $query->fetch_assoc();
if (count($row) >= 1) {
echo "<script>alert('You already add this model')</script>";
}else{
if(!$mysqli->query("INSERT INTO prijzen (prijs, created, model) VALUES (".$prijs.",'".$moment."' , '".$kiesmodel."')")) {echo $mysqli->error;}
else{ echo "het toevoegen is gelukt";}
Laden(0);
}
}
may it can help you