That's an H-bridge - the diodes provide protection for the other arm of the half-bridge in that configuration (which is why the diode is there in the first place), but if you have a single TIP120 its diode does nothing useful.
That's an H-bridge - the diodes provide protection for the other arm of the half-bridge in that configuration (which is why the diode is there in the first place), but if you have a single TIP120 its diode does nothing useful.