In a lab at Caltech Ken Libbrecht has been simulating snowflake formation and under absurdly exact laboratory conditions, they have been able to create almost identical snowflakes.
Clearly, in the real world, we do not observe identical snowflakes because each snow flake has different conditions as it falls from the sky to hit the Earth. Humidity, wind, temperature are not uniform enough to produce "identical" snowflakes. The process is too stochastic. Here is a cool infographic portraying all the different possibilities:
Note: To prevent a straw-man stalemate, I do not propose the snowflakes have to be 100% identical down to the very quark or subatomic level -- I'm not taking an absolutist position. I'm just using "identical" in the reasonable sense. Suppose even after studying two snowflakes carefully for a few minutes, you still can't find any differences.
How can I construct a world such that the humidity, temperature (and other relevant factors that the answerer wants to address) are as uniform as possible?