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I agree to be a little carefull and not to freeze too often. If the boxes close well, there is hardly any need to re-freeze at all, to my opinion.
About destroying DNA-material: if you freeze only one or two times, it may still be okay. After about three years, any (diptera-) material has deminished quality of DNA-material anyway (if I understood correctly; correct me if I am wrong).
So, when used with care, this method is still to be preferred to my humble opinion.

Ciao, Andr