where ‘reference’ is your multiple custom field with the same name.
<?php $reference_urls = get_post_meta( get_the_ID(), 'reference' ); if ( $reference_urls ) : ?>
<h2>Reference</h2>
<?php foreach( $reference_urls as $reference_url ) { ?>
<a href="<?php echo $reference_url; ?>" rel="nofollow"><?php echo $reference_url; ?> </a>
<?php } ?>
<?php endif; ?>