INSERT IGNORE, LIMIT 1... how to check if row exists before INSERT, to make sure it's inserted only once

Question

I've tried both of these, INSERT IGNORE and LIMIT 1 but records continue to be inserted into my DB. Aren't these commands supposed to prevent a new row being inserted if the row exists already ? Is it because the auto_increment value is different when the new row is added and therefore it is technically a different row?

The relevant piece of code looks like this :

 $user_id = 3;
 while ($row = $result->fetch_assoc()) {
            //this the the user_id in the user table of the matching phone number   
            echo $row['user_id']."<br />";
            //call this user_id contact_id
            $contact_id = $row['user_id'];
                $stmt2 = $con->prepare("INSERT IGNORE INTO contacts (user_id, contact_id) VALUES(?,?)") or die(mysqli_error($con));
                $stmt2->bind_param('ii', $user_id, $contact_id) or die ("MySQLi-stmt binding failed ".$stmt2->error);
                $stmt2->execute() or die ("MySQLi-stmt execute failed ".$stmt2->error);

    }

I also tried :

$stmt2 = $con->prepare("INSERT IGNORE INTO contacts (user_id, contact_id) VALUES(?,?) LIMIT 1")

My table structure looks something like this, 3 columns :

auto_inc     user_id     contact_id
   1            3           24
   2            3           12 
   3            3           87
etc...