From: SnowCrystals.com
Now there’s a question I hear a lot. It’s a funny question, almost like a Zen koan — if two identical snowflakes fell, my inquisitive friend, who would know? And can you ever be sure that no two are alike, since you cannot check them all to find out?
Although there is indeed a certain level of unknowability to the question of snowflake alikeness, as a physicist I find that I can address this issue with some confidence. As I will demonstrate, the answer depends to a large degree on what you mean by the question. (Yes, physics does occasionally have its Zen-like qualities.)
The short answer to the question is yes — it is indeed extremely unlikely that two complex snowflakes will look exactly alike. It’s so extremely unlikely, in fact, that even if you looked at every one ever made you would not find any exact duplicates.
The long answer is a bit more involved….