Puzzle mystery

As if this crazy puzzle wasn’t already hard enough!

I was really perplexed about a very bright turquoise blue sky+chalet roof puzzle piece that was leftover after I had finished the entire roofline and blue background areas.

How could that piece be left over? Where could it possibly go?

And then I looked more closely at the finished portion and found an exact duplicate piece already placed!

And then while working on the little people, I found another duplicate piece, part of a little dog playing in the snow.

I have never seen such a thing! I can’t imagine how this happened. It isn’t duplicate everything, but who knows how may others are lurking in the stacks. Now I am worrying whose copy of this puzzle is missing these pieces? And will I end up missing a few random pieces myself? Truly makes an already-difficult puzzle even more so!