Not quite, said Kenneth G. Libbrecht, a physicist at the California Institute of Technology, who found a way to create what he calls “identical twin” snowflakes in his lab.

Since each snowflake faces a different turbulent path through the atmosphere, each twist, turn and fall grants it a unique symmetry. But if you subtract nature’s volatility from the equation, then these icy flowers are no longer guaranteed uniqueness.

By placing two crystal seeds next to each other and growing them under the exact same conditions, Dr. Libbrecht found that he could create two snowflakes with nearly the same intricate shapes and patterns.

https://www.nytimes.com/2016/01/23/science/who-ever-said-no-two-snowflakes-were-alike.html