Have a look at the following links (disclosure: I wrote the second).
At Stack Exchange, the overall goal is to build a repository of answers to questions of interest to specific communities. Placing questions on hold is a way for the community to ask for the question to be improved. Normally, it is only if no improvement is forthcoming within 5 days that the question is closed and possibly deleted.
In the case of a true duplicate, the repository already contains the question, so the duplicate isn't needed. As the question's author, if you think that your question is actually different, the thing to do is to edit your question, linking the purported duplicate, and stating why your question isn't its duplicate, explicitly stating how it is different. You should also take the opportunity to rephrase your question if it helps to clarify exactly what you are asking.
This automatically pushes your question to a review queue, where the community looks at your edits and considers whether to reopen your question. In addition, after editing your question, you can also post a question on Meta to make your case.
Now, to address your specific questions and issues:
What is “an exact duplicate of an existing question”?
It's one where both questions address exactly the same issue. In your case, both questions appear to be asking about the presence or absence of the word "to" in a list of conjunctions. If your question is different, you should explain how it is different.
I urge you to consider more about learners point of view before marking a question as a duplicate.
Determining whether a question is a duplicate is normally fairly objective process. Reviewers look at both questions to see if there is so much overlap that the new question doesn't add anything to the repository. If there is, and if the other question has been answered properly, the learner loses nothing by looking at the answers to the other question.
Note that before asking questions, users are expected to check the repository for answers. The goal of SE is to build a repository of expert answers, and the repository already contains many questions that are curated to be on topic and well-answered, so it's a good place to check.
I also would like to suggest some kind of mechanism in a question page marked as a duplicate which enables questioners to ask the reason why they got the label.
This can be done in comments under the question, or on Meta.
I believe people who [place] a question [on hold] have some responsibility to the questioner because they have taken away a chance of getting answers from the questioner.
In the case of a true duplicate, the answers should exist on the linked question, or others can navigate to it via the closed question, and place new answers there.
Where it isn't a true duplicate, the wording of the question would have already misled (typically 5) others. If the question was kept open, the answers you get might not suit the question you had in mind. Editing to clarify your question helps you to get better answers.