From How should duplicate questions be handled?:
When are two questions considered duplicates?
According to Joel Spolsky, we should only close real duplicates, and according to Jeff Atwood, there are three kinds of duplicates: cut-and-pastes, accidental duplicates, and borderline duplicates (requiring judgement as applied by the community).
Questions may be duplicates if they have the same (potential) answers. This includes not only word-for-word duplicates, but also the same idea expressed in different words.
Questions asking about the same aspect of the same concept, but with different examples, may or may not be considered duplicates. It depends how easy it is to figure out one example from the other. If it's only a matter of changing some numerical values or some variable names, they're duplicates. If understanding why the questions are at all related requires a detailed explanation, the questions aren't duplicates, merely related.
From The Wikipedia of Long Tail Programming Questions:
If you’re going to close a user’s question as a duplicate, it has to be a real duplicate. For example, if a user asks, “What does the IP address 22.214.171.124/24 mean?” it’s OK to close that as a duplicate of a more general question like “What do IP addresses of the form a.b.c.d/e mean?”
So is How does a Battle Master's Parry work with multiple damage types? a strict subset of What part of a multi-type damage roll is reduced by a non-type-specific effect?
To my mind, yes it is and should be closed as a duplicate.