As a child, I was told that every snowflake is unique. Indeed, I'd seen microscope photos of snowflakes, showing their beautiful geometric designs, and I assumed it was true. But of course, as with all things, science ruins a perfectly good blanket statement.
In the video below, Deep Look visits a CalTech lab where professor Ken Libbrecht grows snowflakes. Using timelapse photography we see how snowflakes form, and how minute differences in the atmosphere affect their shapes. What's surprising is how Libbrecht can control the lab environment so precisely that "twin" snowflakes sometimes form.
So while the vast majority of "wild" snowflakes are unique, not all snowflakes are. Here's proof: