为什么Mozilla会插入重复的行？ PHP / SQL

Question

So, i got this code:

<?php
    $servername = "localhost";
    $username = "**";
    $password = "**";
    $dbname = "TestDB";
    // Create connection
    $conn = new mysqli($servername, $username, $password,$dbname);


    // Check connection
    if ($conn->connect_error) {
        die("Connection failed: " . $conn->connect_error);
    }
    echo "Connected successfully<br>";

    $ip = $_SERVER['REMOTE_ADDR'];


    $sql = "INSERT INTO testdata (id,address,count) VALUES (DEFAULT,'$ip',1) ON DUPLICATE KEY UPDATE count=count+1";


    if ($conn->query($sql) === TRUE) {
        echo "New record created successfully<br>";
    } else {
        echo "Error: " ;
    }


    $conn->close();
    ?> 

With this code ip-addresses of customers are saved and it should increase count by 1 every time they return to the website. When i visit database.php directly it works like it should in every browser, but when i call the php-file via the index.html-page it counts up twice (most of the times, not always) in Mozilla, other browsers no problem, the code in html-file:

<img style="display: none;" src="http://servername/database.php?">

Anyone know why? Please help, i'm really stuck here