I was under the impression that "marked as duplicate--this question already has an answer" meant that OP's question had not only elicited an answer or answers, but elicited one that satisfied the OP and had been officially selected. But apparently that is not the case, so what constitutes an answered question?
Users with at least 3000 rep can vote to close questions. One of the options to do so is
This question has been asked before and already has an answer.
However, a duplicate is not necessarily an exact copy. An exact copy is merged, not closed as a duplicate.
A question nominated as a duplicate has at least one answer which provides an answer to the closed question. It may not be the accepted answer there, but one of the answers will either answer the question, or show the way to answering it.
The nominated question may be a generic question like Are there any simple rules for choosing the definite vs. indefinite (vs. none) article? — that one will show the asker how to determine which article they should use, in order to answer a question about articles. That generic question can't answer every specific question which is linked to it as a duplicate, but does provide an answer which can be applied to the closed question.
In addition, the closed question becomes a pointer to the canonical answer, which (if the system is used correctly!) will quickly refer people with similar questions to their answer.
The asker of the closed question can comment on closure suggestions during the time votes are being amassed, because there is an automatic comment advising them of a potential duplicate, and can still do this after it's been closed. They are at liberty to comment (or, preferably, amend their question) in order to show that the nominated duplicate is not appropriate.
You are right that what the current closure system does not do is to link directly to the best answer within the "target" question.