During WW1 the German ambassador in Washington, von Bernstorff, blamed their numerous transmissions of the same messages as the cause for the successful decoding done by the British. I don't see the reasoning unless the same messages were sent in different codes, but why would the Germans do that? Bernstorff said:
There is absolutely no cipher which they (the British) cannot decipher, provided they have before them a sufficient amount of telegrams. And this result, particularly in the case of the United States, was probably due to the fact that circumstances were such as to force us to make use of an extraordinarily large number of ciphered messages, and we often sent our reports and telegrams in double or triple form, in the hope that in some way they should reach Germany. Consequently, the British must have had an enormous amount of material in the way of cipher dispatches of ours, and in this way it was possible for them to break down our various ciphers.