Not all snowflakes are unique in their shape. There's one fact for you.
And here's another: The shape of snowflakes — whether individually distinct or mass-production common — is determined by chemistry. Specifically, the shape is a function of the temperatures and meteorological conditions the snowflakes are exposed to as they form and the way those factors affect the growth of ice crystals.
This short video from Bytesize Science will give you a nice overview of snowflake production and will help you understand why some snowflakes are unique, and why others aren't.