I completed a Zwift Fondo event today that began at 9am UTC on 30th December 2022.
On completing it, I got awarded twice for completing both the December and January Fondos even though I had only done a single event.
Since this is a Zwift server-side problem, my Zwift setup presumably not relevant. Guessing this is widespread/happened to everyone. Not clear which I should have been awarded for (December or January) but certainly not both.