When I took statistics I was taught that to draw a valid conclusion one had to take a representative sample of the entire population. They didn't, of course.
Chances are, in fact, that there are lots of duplicates.
What the snowologists really mean is that your chance of finding duplicates is virtually zero. It's been calculated that in a volume of snow two feet square by ten inches deep there are roughly one million flakes. Some of these are surely repeats. On the other hand, a single snow crystal contains perhaps 100 million molecules, which can be arranged in a gigajillion different ways. By contrast, the number of flakes that have ever been photographed in the history of snow research amounts to a few tens of thousands.