Need help to hide Duplicate Content on Homepage

I need help to hide duplicate content based on tag.

I like to remove the tag (let say with tag id -15) from showing up.

What do I add here?

here's the code

		<?php $count = 0; ?>
		
    	<?php if (have_posts()) : ?>
    	
			<?php while (have_posts()) : the_post(); $count++; ?>

Code (markup):

please help me modify this code so it doesn't show posts with the tag-id 15
thank you very much!