To reproduce:
1) download the project attached in https://forum.unity.com/threads/xbox-one-controller-buttons-fire-multiple-times-only-when-two-controllers-connected.580480/#post-4117315
2) Make a UWP build and deploy to Xbox
3) Connect one controller to the Xbox and press any button to see how the background image disappears and reappears on subsequent presses
4) Connect a second controller and press any button on any of the 2 controllers again
Result: Now multiple inputs are read at the same and the background image blinks rapidly.

Note: Not reproducible using the Xbox Unity module

Reproduced on: 2019.1.0a14, 2018.3.3f1, 2018.2.20f1

Note: This bug impacts UWP apps in general and not just on Xbox. It's specific to reading the "global" keystate, i.e. Joystick 0, when multiple Xbox gamepads are connected. Since the UI Button's default OnClick() event is triggered from any input source (including Joystick 0) it will repro this bug, but reading input directly from a given gamepad should work fine.