Duolingo

I saw it twice. The first time I didn't actually get extra XP, the second time I did.

Also, if you get a second multiplier while the first is still active, now it just adds itself to the end of the multiplier's active time instead of stacking.

Never

I got one recently when I opened all three of my daily chests. I had the bonus from the first two still active at the time. I was already falling asleep so I never used it.