Main Menu -> Copy problem

can not see "Main Menu -> Copy" Under Menu manager have only new option?? Im in Version 1.7.3

www.mobilejoomla.com/forum/3-general/6726-no-option-to-copy-menu-in-joomla-17.html

Hi, I have followed the instructions for making a copy of the main menu.  However, when I do this I get unexpected results on the desktop version of the site. Modules disappear on pages.  Does anybody know what causes this?

If I delete the mobile_menu everything works again. 

I'm using J1.5.22