They're different, but perhaps only subtly so
(Full disclosure, I know the answer to the closed question and fully intend to answer it should it get reopened.)
The (current) target is essentially asking: From only knowing the name, what is the "PHB+1" rule. The answer to such a question is expected to be where to find the rule, and generally how it works.
The (current) dupe is asking about the nature of a specific part of that rule. It already knows where to find it and how the rule generally works. As such it is similar to other questions we have (non-exhaustive list) which are asking about specific problems with the "PHB+1" rule:
While the general answer could give a clarification so as to answer the specific problem, that would
expand the scope and length of the answer substantially,
clutter the answer with detail that isn't of relevance to many other readers,
Bury the answer to the specific question in the general one, making it equally hard for someone with the specific problem to find that answer as it is to figure it out themselves.
and there is no end to the possible misunderstandings one could have with the rule, multiplying the above rule.
So, keeping it closed as a dupe isn't benefiting either question (or its answers), as such it shouldn't be closed.