Doubling them up seems logical. If you did prefer not to do that, you could differentiate between the + / - on the main keyboard (those next to the regular digits above alpha keys) and the + / - on the numeric keypad.
For those with keyboards having no numeric keypad, these are usually made up from a key combo of a Fn (function key) and another key.
The ultimate solution in my mind however, is using an INI file (or other methodology) to allow us to set / change the hotkeys for the editor itself.
