Interesting that each of the responses so far, to the posting, have assumed that the OP (i.e. original poster) should take specific actions.
To me this ignores the real problem which is that there is no quality control constraints on MathSE reviewers labeling a question as a duplicate.
Here, I am confining attention to those questions that are not otherwise regarded as low quality. In my direct observation, MathSE reviewers tend to get frustrated by the fact that the vast majority of questions are of low quality. Consequently, all too often they will do one of the following:
Label the question as a duplicate, referring the OP to a previous MathSE question despite the fact that it is very plausible that the OP will not be able to understand the answers in the previous question.
Label the question as a duplicate, but refer the OP to the wrong previous question. That is, the referrer will not take the time to ensure that they are referring the OP to the correct previous question.
Label the question as a duplicate when the question is very similar to a previous question but where the question has a unique subtle nuance that renders the question not a duplicate.
This has happened to me, and I have seen it happen to others. Often it is because the OP is long-winded. Personally, while I think that it would be nice if the OP had the artistic skill to be brief, I can't see penalizing them for lacking this skill.
Sometimes it is because the MathSE reviewers are narrow minded. Also, from what I have observed, once the MathSE reviewers emotionally dig their heels in, declaring the question as a duplicate, it is game over. They simply will not change their minds.
Unfortunately, all that I can offer is criticism (which is easy) and no concrete remedy to consider. For example, preventing MathSE reviewers from answering low quality questions is itself very problematic. Therefore, how can one expect MathSE reviewers to accept quality control standards with respect to labeling a question as a duplicate?