Being prompted twice for shutdown?

[Gilles Gravier]

Hi!

On some of my machines (not all), running Ubuntu GNOME(15.10) when I ask
the machine to shut down, I get the [Cancel] [Restart] [Power Off]
window... when I select Power Off, that same menu disappears then
reappears with the same prompts. I select once again Power Off and the
machine properly powers off.

Any idea why this double prompt?

Cheers,
Gilles