query in category.php repeats itself

You should take the condition outide the loop meaning above

<?php if (have_posts()) : while (have_posts()) : the_post(); ?>

Therewise every time the loop begins it start the check instead of doing the check then starting the loop if the condition exist.

Its hard for to understand because i dont understand div names
but i think only should be inside the loop

                        <div id="contentholder">
                        <div class="uitleg">  
                            <?php the_content(''); ?>
                        </div>
                        <div id="uitlegdivider"><img src="https://wordpress.stackexchange.com/questions/31814/<?php bloginfo("template_directory'); ?>/images/uitlegdivider.png" alt="" width="26" height="171" /></div>
                        <div class="voorbeeldimage">
                            <img src="<?php echo $drukproefimage; ?>" />
                        </div>
                    </div>

Explanation:
the loop = if (have_posts()) : while (have_posts())……

Anything iside that would go again and again until there are no posts fitting the condition you set before and not inside it.