Today we have new question - or maybe not
If part of what you're saying there is that it's not totally clear whether this is a dupe or not, I agree! One of the questions is 'where do I start with learning theory if I want to produce music in FL'; the other is 'I'm a guitarist but I don't know how to gain an understanding of musical theory'.
Is a good answer to one of those questions necessarily a good answer to the other? I don't think so.
Could an answer to one of those questions be a good answer to the other? Yes, I'm pretty sure it could.
In this particular case, are the answers to How to learn musical theory? genuinely good answers to What to learn first? ? I'm not sure they are. Although I totally agree with the idea of pointing to good canonical answers, I'm not sure the question pointed to in the "already has an answer" notice here is asked, or answered, from a general enough perspective to be a good canonical answer. I personally wouldn't have closed as a dupe of that question, even though I totally agree that similar questions come around quite often.
Should we have a stock answer for this? (It's really too broad a question anyhow.)
I think you've captured the problem - as a site, we seem a little unsure whether these kind of broad-brush-strokes beginner questions are OK, of if they're too broad, or too opinion-based, or what. So people don't really take a lot of effort in answering them (after all, if you start writing a long answer, it's liable to be closed before you submit it!) and we never really get those good canonical answers.