If your MIDI keyboard is also a sound generator, you will likely want to stop the device from generating its own sounds while you are using it with MainStage, to avoid doubling notes between the device and the MainStage patch you are playing.
Most MIDI synthesizers and other MIDI controllers with tone generation capabilities include a function known as Local Control. By turning off this function, the device’s internal tone generation is suppressed.
On the device, turn on the Local Off function.
If you can’t find the Local Off function in the MIDI menu of your keyboard, consult its manual on sequencer use. Some keyboards allow you to select from Local, MIDI, or Both for each of their Parts (individual MIDI channels/sounds in multitimbral MIDI devices). The MIDI setting, if applicable to your keyboard, is the equivalent of Local Off.