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A typical snowflake weighs something like a miligram. That's about 6E20 hydrogens. About i in 6000 of those are heavy hydrogen, so picking hydrogens randomly as you assemble the snowflake you have a (roughly) 5999 out of 6000 chance of picking the right one. Sounds like pretty good odds until you think about doing it 600000000000000000000 times. I'm sorry but my calculator won't handle numbers that small. That gives the chance of getting the hydrogens correctly placed. Now think about the fact that of those vanishingly small number of snowflakes with the hydrogens in the right places, a similarly small fraction will have the oxygen isotopes correct.Of that tiny fraction of a tiny fraction, only those that are the right shape (again a tiny fraction) are identical.Strictly, that doesn't prove that there are not 2 the same, but it does give some idea of how unlikely it is.