I don't have a Duet, but I think that is normal. The LED indicating that the signal line is shorted to ground when the switch is untriggered/carriage is away from the endstop, and pulled up (light off) when the switch is triggered. See [
github.com] and [
reprappro.com] portion about the lights on the board for the description of the normal operation of the Y-endstop light.
I like that the light is on in the untriggered/NC state and toggles to off when the stop is triggered/opened.