php mysql ช่วยดู ให้หน่อยครับผม อยากจะเช็คค่าที่ซ้ำกันไม่ให้บันทึกลงฐานข้อมูลได้

$id_teachers = $_SESSION["id_teachers"];

//print_r($_POST["id_topic"]);
$id_topic = $_POST["id_topic"];

//echo implode("','", $id_topic) . '<br>';




$sql3 = "SELECT * FROM topic_interest ";
$result3 = mysql_query($sql3, $c);

echo $sql3."<br>"; 

while ($row3 = mysql_fetch_assoc($result3)) {
    $aa[1]=$row3["id_topic"];
    $bb[1]=$row3["id_students"];
    
    print_r($aa)."<br>";
    //print_r($bb)."<br>";
    
    
}

$sql2 = "SELECT * FROM topic 
        WHERE topic.id_topic IN ('" . implode("','", $id_topic) . "')";
$result2 = mysql_query($sql2, $c);

echo $sql2."<br>"; 


//echo implode("','", $id_topic) . '<br>';
//echo $sql2 . "<br>";
while ($row2 = mysql_fetch_assoc($result2)) {
    echo $row2["id_topic"];
   /*if($aa!==$row2["id_topic"]){
      
    $sql = "insert into topic_interest (id_topic,id_teachers,id_students) 
            values (" . $row2["id_topic"] . " ,'$id_teachers'," . $row2["id_students"] . ")";
    $dbquery = mysql_db_query($dbname, $sql);
    echo $sql . '<br>';
       
       // echo $row2["id_students"]."<br>";
        
   }  else {
   echo $row2["id_topic"]."ซ้าแล้ว";    
   } 
    */
}