How allowed "--", "---" or any other symbol in terms(category) URL. Now WordPress replace from "--" to "-"
ps.I`m using to Subdomain plugin. And IDN subdomain Example: xn--...site.com/ There xn-- is category (url)
The -- to an mdash conversion happens inside wptexturize() http://codex.wordpress.org/Function_Reference/wptexturize which is applied to many things (including taxonomy titles and other functions output.
Look for places you might want to remove the filter from in wp-includes/default-filters.php and remove the filter with the remove_filter() function: http://codex.wordpress.org/Function_Reference/remove_filter