Joomla 3.6.3 + bootstrap 3.3.6 with a resolution of 1200 and below, duplicate content appears in the modal window. Why?

Question

Joomla

I'll start with a basic description of the problem. For the first time I had to work with CMS Joomla. The task, in my subjective opinion, was very simple and consisted of literally two points to make up a template and actually pull it on Joomla. I used to work with Modx and such tasks were literally one day's work. However, with Joomla, I got stuck.
One of the problems is duplicate content when opening a modal window. Those. instead of, or rather even on top of, the modal window form, the entire site is superimposed along the width corresponding to the size of the modal window. At the same time, everything is ok at full-size resolution, but if the browser window is "shrunk" to the next bootstrap breakpoint, for example, to 991px, then this problem arises. Please advise how to solve this problem. I would be glad for any hint including links to good tutorials on joomle 3.+.

Answer 1

Missing code fragments from the comment
Here you are interested in the lines about jquery. Insert before
Output module with minimal framing and h3 header is removed in module settings, setting display header.

<jdoc:include type="modules" name="right" style="xhtml" />