They are all duplicates
The way we treat duplicates currently is outlined in this Meta:
We close Question A as a duplicate of Question B when all of these are the case:
- It's the same question, or Question A is already covered obviously as a subset of Question B. Obvious here means I can tell at a glance it's there.
- Question B has an obvious answer to Question A. Obvious here means I get a straightforward answer without hard searching — a couple
of sentences buried in the middle of a post, or an answer which only
sort of implies an answer to Question A, doesn't count as obvious.
- There is not some strong compelling reason to covering Question A alone, separately from Question B. (If the above bullet points are met
this rarely happens.)
Depending on what question is considered to be the duplicate (see below for my recommendation), this set of questions easily meets all three bullets of the above criteria for closing as duplicate. Specifically, the answer to each of these questions is a quote of the same simple rule that applies to all such cases. Thus, they do meet our criteria for duplicate closure and there is no compelling reason to leave them separate.
Conceptually, each one of them is asking about some feature or features and how it stacks with Expertise. We know the answer to this and it is a general rule that covers all such cases. As such, there is nothing about any feature that could give us a different answer besides referencing this rule. And indeed all the answers simply reference this rule. Thus, they are duplicates because their core issue is the same and the only thing that is different is the minor details which don't affect the answers at all.
The most general one is probably the best dupe target (duplicates should be reversed)
Do features that allow you to add twice your proficiency bonus, under some other name, stack with Expertise? is probably the best Q&A to dupe the others to (full disclosure I do have my answer on this one). This is because it is the only one to ask about the concept in general and not about specific features. It gives an example in the text but the question itself applies to all features that might stack with expertise. As such it is an umbrella that could cover all the other questions. This aligns with the first bullet point of the previous quote easily making all of the other questions a subset of this one.
That being said, a simple one line edit to any of the other questions would give the same effect (though at the cost of slightly changing the querent's intent).
Future Q&As will be judged on their own merit
Just because all of the current Q&As that you listed should be duped together does not mean that all future questions will be. There could be a feature that does not apply to this rule or a new rule or feature could come out which changes things. Either way, we need to apply expertise when judging closures to understand what the actual core of a question is.