I also encountered this today with a planeswalker on the battlefield. Had Calamity, Galloping Inferno saddled and attacking with an Annie Joins Up on the battlefield and I think the initial trigger triggered twice (due to Annie Joins Up), but the "Repeat this once" triggers didn't - I got 2 copies instead of 4.
I also encountered this today with a planeswalker on the battlefield. Had Calamity, Galloping Inferno saddled and attacking with an Annie Joins Up on the battlefield and I think the initial trigger triggered twice (due to Annie Joins Up), but the "Repeat this once" triggers didn't - I got 2 copies instead of 4.