Hi,

With the current org-do-latex-and-related function, fontification of
a region before a LaTeX fragment repeatedly adds the
'org-latex-and-related face to the fragment.

You can reproduce with an org buffer with the following content.

#+BEGIN_SRC org
Foo
  $a^2 + b^2 = c^2$
#+END_SRC

  - Use (setq org-highlight-latex-and-related '(latex))
  - Write some text at the end of the first line, triggering
    refonfication of the line
  - For each character inserted, the org-latex-and-related face is
    added (as a duplicate) to the fragment, as you can check using
    describe-text-properties.

The attached patch fixes this.

-- 
Sébastien Miquel