It is not a duplicate and should remain open.
Question A: If I have the fighter's Extra Attack feature and the warlock's Thirsting Blade eldritch invocation, how many attacks can I make?
Question B: Does this strict reading of the rules allow both Extra Attack and the Thirsting Blade warlock invocation to be used together?
We will examine these questions according to these three duplicate criteria.
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.
In this case, Question A asks simply, "How many attacks do I have if I have Extra Attack and Thirsting Blade?" Question B, while very similar, is a substantively different question, which can be summarized as, "If I have Extra Attack and Thirsting Blade, do the Two-Weapon Fighting rules introduce a technicality that allows me to take advantage of both features?" These are two different questions. One asks if Extra Attack and Thirsting Blade stack, one asks specifically about how the two interact with Two-Weapon Fighting.
But is it a subset situation? I think not. Question B (correctly) assumes that they normally do not stack and goes on to ask if a particular scenario presents a loop hole to this normal behavior. Question A is asking what the normal behavior is, so we don't have a subset situation. For an example of a "subset closure", see this question and its associated meta discussion.
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.
Question B has an obvious answer to Question A. The answer to each question is the same, despite the questions being different.
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.)
They are substantively different questions. It is not clear from Question B that it is being asked if Extra Attack and Thirsting Blade stack in general, as the focus of Question B is on their interaction with Two-Weapon Fighting.
Since Criteria 2 is the only one that is met, we conclude that Question A should not be closed.