Addressing the Other Answer Here
With five votes in favor of it, it places a significant burden of doubt on me, so I feel it is necessary to comment upon it:
"They seem the same to me. If there were any situations where you’d prefer one to the other, the answers to the famous question would attest to that. Which is the test for duplicates: would the answers to one also answer the other? Note, the test is not whether the current answers do so, but that the question invites answers which could."
I disagree with this assertion for a few reasons. The first is that many people take a minimalist approach when answering questions, and only address what is explicitly asked of them. Indeed, a question can be closed for having too broad of a scope.
Another is that a perfectly adequate answer such as NoHat's vaguely discourages more extensive answers on the grounds that the principle matter had already been addressed
Finally I think we can take the fact that none of the older answers, except the one Mari-Lou specifically commented upon, addressed this answer as a sign that it does not really seem to invite this sort of answer
I am not Suggesting that such an answer would be clearly disqualified from the questiion in its present form, but I do not think that is adequate in and of itself to merit closure. In the next section, I shall explain why in the following segment:
I Don't Think This Question Should Be Closed As a Duplicate of That Older One
If this was a matter of what could necessarily be interpreted as a broader and narrower question, I might support closing the narrower question as a duplicate of the broader one, but I do not think that is really the case. I think this is a question of addressing fundamentally different scopes.The old question is about commodity or correctness of one form over the other, with the answers addressing regional usage frequency and is postulated on the notion that they effectively mean the same thing. The new question is about differences in meaning. That is a significantly different scope in my opinion. Now differences in meaning could be used to argue that both forms are appropriate, but originally, none of the answers to the old question interpreted it that way and none of their answers would qualify as answers to the new question.
According to Stack Exchange Meta duplicate guidelines, the small amount of overlap does not necessarily mean that your question should be closed. At the very most, your question is at most, a borderline duplicate:
According to Spolsky we should only close real dupes, and according to Atwood, there are three kinds of dupes: Cut-and-paste, Accidental duplicates, and Borderline duplicates (requiring judgement as applied by the community.)
There are a few factors which should go into deciding which question is a duplicate based on the fundamental goal of duplicate closure mentioned in Why Are Some Questions Marked as Duplicate:
The fundamental goal of closing duplicate questions is to help people find the right answer by getting all of those answers in one place. This does not mean that every duplicate will immediately be closed; we love (some) dupes. There are many ways to ask the same question, and a user might not be able to find the answer if they're asking it a different way.
One is is that duplicate status should make it easier to find the answer people answering the question would seek. This is such an important criterion that it is not even possible to close a question as a duplicate of another question that has not yet been answered, even if it is indeed identical. Moreover, the answers to Mari-LouA's question imply that both are indeed proper, but the opposite is neither necessarily nor currently true. MariLouA's question is of a relatively reasonable scope that makes it easier to find specific semantic arguments
The other is that duplicate targets should not be chosen on the mere basis of age, but also on the basis of answer quality, because we want to be directing people to the best set of answers. On these grounds, if one question must be closed as a duplicate of the other, I would prefer to be directing people to MariLou-A's question, which has much more informative answers overall. Given that the other question is not a dialectical question, the answers to the other question are primarily opinion based in my assessment, with the exception of Nohat's, which has its basis in verifiable usage frequency facts. Now yes, the older question has more votes which normally implies higher quality contributions, but it also has the advantage of more age and more views which give it the natural propensity of having more votes. When accounting for the proportion of time and views both questions have, I think it is clear that the answers to Mari-LouA's have been better received overall.
I think another solution should be implemented
In this case, I think we have enough clues to deduce what the original questioner asked was meant to be different, so we can make a clarifying edit on behalf of the questioner that would cleanly divorce these questions. The questioner presented usage frequency facts from a questionable source, and accepted an answer based on more reliable usage frequency data. I propose that we change the older question's title to:
Which Is More Common Between Fill-In and Fill Out?
This has two benefits for us. One is excluding answers based on semantics, corralling them over to MariLou-A's question, or potentially a less restricted question about a more widespread distinction if we ever get one in the future. Another is that the extra keyword makes it easier for future researchers coming in from google to find the frequency data Nohat presented to us.
Most of the answers seem like they would remain equally valid with this change in wording too, except maybe Alan's "Both are perfectly acceptable" which could probably be changed to equally as suggested by a comment. I am not really too concerned about preserving the validity of an unfounded one liner anyway.
If the questioner rolls back, and/or makes a clarifying edit of their own which contradicts these reasonable assumptions, then perhaps a reassessment would be in order, but given what we know now, I think this is the most helpful course of action for all of our future visitors.
Perhaps I am wrong on that count, and I would welcome an audit of this opinion if it is too presumptuous. However, if it is so, then I am not sure that we can reasonably assume to know what type of answer the questioner wants, and perhaps the older question should be closed as being unclear instead, in preference of this question which provides a more obvious goal, (alongside a better research effort and so on).