Stop duplicates in dropdown box

This code stops duplicates appearing in your dropdown boxes.
";
$sql = "SELECT tagname FROM tags order by tagname"; // new options mysql
$resulta = mysql_query($sql);
echo "" . $row['tagname'] .""; //or put whats in the database in select options
while ($rowa = mysql_fetch_array($resulta)) {
//prevents dupes in dropdown
$tagname= $rowa['tagname'];// get all results
if($tagname != $row['tagname']){ //filter results
// end of prevent dupes

echo "$tagname"; //output filtered results
} //end while
} //end while
echo "";
}
?>