Index of same values in Tkinter Optionmenu widget
This isn't how the option menu is designed to work. You'll have to create a custom option menu. An option menu isn't anything special, it's just a button and a menu, and a couple of functions to give it the behavior that it has.
From a usability perspective this is a very bad design, since the user has no way of knowing the difference between the first "b" choice and the second "b" choice. I hope one choice doesn't map to "deactivate the bomb" and the other is "explode the bomb".