Simple quiz - Check if users have already participated

[Challenge]

Hi all.
I need a competition/quiz script.
Doesn?t need to be anything fancy, so here's a description:

Each member can only participate 1 time to avoid cheating.
Radio buttons shall be used as answers.
Then users have to type in their name, address, zip-code, city and age.

If the user answered correct, then the results should be inserted into mysql table com_true.
And if the user answered incorrect, the results should be inserted into table com_false

The main issue for me is to do so that every user can only be able to participate 1 time...

Hope you understand!

If i expressed myself in a bad way, please let me know!

Regards,

Challenge.

Instead of inserting, it can just be sent to my mail adress...

If its possible, you can use this to build it up from:

<?
if (isset($_POST["name"]) && isset($_POST["mail"]) && isset($_POST["answer"])) {
// ## your email ## //
$adminmail = trim("mail@mail.com");

$msg = "Navn: $_POST[name]\r\nEmail: $_POST[mail]\r\nAnswer: $_POST[svar]\r\n";
mail($adminmail, "Answer", $msg);

header("Location: ".$PHP_SELF."?action=sent");
} elseif ($_GET["action"] == "sent") {
echo "Thanks for the answer";
} else {
?>
<form method="POST" action="<?=$PHP_SELF?>">
Name: <input type="text" name="navn">
Email: <input type="text" name="mail">

Answer:
<input type="radio" name="svar" value="1" checked> Svar 1
<input type="radio" name="svar" value="2"> Svar 2
<input type="radio" name="svar" value="3"> Svar 3

<input type="submit" value="send">
</form>
<?
}
?>

Challenge