When one includes a template more than once on a page, only the first include is displayed properly.

Simple testcase:
1) Create a new html template file with some text
2) Include it twice or more times in a row in an existing template of your choice (index_body.html for instance)
3) Clear the cache just to make sure
4) Reload the page

The page does show the included content only once, no matter how many times you included the template.

Interesting side note: when you edit the included template file, it will get included and displayed properly once (at least for 2 includes, not for more), until you reload the page and it disappears again.