single.php fires more than once after clicking on any post to view with different post id each time

I am answering this because nobody yet answered this and I think this could help others who are experiencing this kind of issue.

As mentioned by s_ha_dum that the issue is related to FireFox’s Prefetching mechanism. In order to resolve this I inserted the below line to function.php:

remove_action('wp_head', 'adjacent_posts_rel_link_wp_head');