Exercise 2: How the Duplicate-Copy Problem Differs From Chapter 7's Decay — Possible Solution
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WHAT CHAPTER 7's DECAY PROBLEM WAS
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Per Chapter 7, decay happens when the original, single, authoritative
copy of a document silently stops matching reality - the underlying
system changes (a server is decommissioned, a procedure is updated)
while the document itself, sitting in its one canonical location,
never gets updated to match. There's exactly one document, and it
becomes wrong over time.

WHAT THE DUPLICATE-COPY PROBLEM IS INSTEAD
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Per this chapter, "once a document is updated, old copies pasted
elsewhere - a chat message, a different wiki page, a printed page left
on a desk - don't get updated along with it." Here, the original,
canonical document is genuinely current and accurate - it was
successfully updated. The problem is that a second, independent copy
exists somewhere else, and that copy didn't receive the same update.

WHY THIS IS A GENUINELY DIFFERENT MECHANISM, NOT JUST DECAY BY ANOTHER
NAME
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In Chapter 7's decay, the failure is a missed update to the one
document that matters. In the duplicate-copy problem, the update
actually happened correctly - the failure is that a second, unofficial
copy exists at all, outside the update process entirely, and nobody
updates it because it isn't recognized as something requiring
maintenance in the first place. Fixing Chapter 7's own countermeasures
(ownership, review cadence, tying updates to change processes) would
keep the original document accurate perfectly well, while doing
nothing at all to fix or even notice the stray copy sitting elsewhere.

WHY BOTH STILL PRODUCE THE SAME PRACTICAL OUTCOME FOR THE READER
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Per this chapter, "the original document can be perfectly current
while an outdated copy of it, still findable somewhere else, actively
misleads whoever happens to find that copy first." A reader who
stumbles onto the stray copy experiences exactly the same failure a
reader following a genuinely decayed original document would - wrong
information, presented with the same apparent authority - even though
the underlying cause and the correct fix are completely different.

WHY THIS WORKS AS AN ANSWER
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It explains what specifically goes wrong in Chapter 7's decay (the one
canonical document silently becomes wrong) versus the duplicate-copy
problem (a second, unofficial copy exists and was simply never
included in the update at all), and explains why both nonetheless
produce an identical bad outcome for whichever reader happens to find
the wrong version.