It is not sufficient to blindly apply the DRY principle.
Where the single representation ends up living is just as important as avoiding the duplication in the first place.
Having the single, unambiguous, authoritative representation of a piece of knowledge living in a place that it doesn't belong is only slightly better than having it duplicated, once in a place where it seems to belong, and once in a place it does not.
So included in the task of reducing (or avoiding) duplication is the consideration of where to put the common code, *including* the task of inventing a new place if it doesn't seem to fit anywhere, now that it's being used from multiple places.
Usually, a new place is waiting to be invented -- that is why there was duplication in the first place -- the place didn't exist, or else it would have been fairly obvious where to put the logic and how to reuse it in the first place.
Perhaps that is part of what "authoritative" means.
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