Uncertainty as to the line between "one piece of information" and "two pieces of information" may be a problem. I don't think it makes sense to say it's "the problem" when most people don't know that DRY is formulated in those terms in the first place.

Personally, I don't think the ambiguity is actually much of a problem; often it's not ambiguous, and when it is it's usually the case that multiple ways of organizing things are reasonably appropriate and other concerns should dominate (they may need to anyway).

I read your second paragraph as vagueness is fine, which sort of makes DRY not a helpful principle but a handwavy problem statement with no clear anything.

As in most vague problems, two extreme solutions (join vs dup) are a wrong way to think about it. I have some ideas on how to turn this into a spectrum in a nearby comment.

I think it is important because DRY-flavored problem is basically the thing you meet in the code most. At least that is my experience, as a guy who hates typing out and rediscovering knowledge from slightly different code blocks or tangled multi-path procedures and refactoring these — either in hope that nothing breaks in multiple places, or that you won’t forget to update that one semi-copy.

I’m programming for a very long time and seemingly no one ever even tried to address this in any sensible way.