[theme: own design] Issue with 'the_post()' included in single.php

I designed some categories differently, thus I have also a series of design variants on single.php. (It’s not complicated what I do, just another color for the navigation bar and some other details)

For the ease and to keep everything clean, instead of always repeating the whole identical code for ‘the_post()’, I put this in a file apart that I included in single.php.

Everything went fine until I installed plugin ‘nrelate’. Because of that include I got a double output of related posts in my single.php file. When I replace it back with the original code, the second series of nrelate’s image links disappears just by refreshing.

I would like to use a plugin as ‘nrelate’ and I would also like to go on including ‘the_post()’ code.

Thus, or I need a resembling plugin that doesn’t double the code, or ‘single.php’ has to be a little more adapted, but I don’t know how and don’t see where to begin. I never made a plugin neither.

My site is http://boostypress.com. But you won’t see very much in the source as I’m talking about php code.

Here two posts to show the visible difference (scroll down until after the end of the post)

1. Example post in category ‘fashion’. Here it is the original ‘the_post()’ code in single.php. (I say single.php but in reality they all have a slightly different name also):
http://boostypress.com/blog/2014/01/20/40s-vintage-fashion-lbd-from-the-forties/

2. The single.php for ‘real estate’ still contains the include and has thus double nrelate output:
http://boostypress.com/blog/2014/01/21/state-london-property-investment-market/
I can leave it still unchanged but maybe not for longer than a couple of weeks.

If there is a need that I test it on a regular theme, well if I really have to, but it is quite some work to make different styles for different categories. I found the solution for ‘different styles for different categories’ on internet, but never read about an include of ‘the_post()’ code into ‘single.php’, that’s rather strange.