You'll say farewell twice some day (2,3,2)

I believe the answer is:

by and by

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'some day' is the definition.
Both the answer and definition are adverbs. Perhaps they are linked in a way I don't understand?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for by and by that I've seen before include "Eventually" , "Future" , "Before long" .)

I've seen this clue in the Sydney Morning Herald.

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