If the question is a duplicate of an existing one, I would vote to close the new question, and answer the old question, if I am able to give an answer without saying what already said in other answers, or adding more details to what already said in the existing answers.
The fact the old question doesn't have an accepted answer doesn't mean the new question is not a duplicate. The fact a question doesn't have an accepted answer doesn't mean anything, except that none of the answers were helpful to the OP, or the OP forgot to accept an answer, which could happen when the OP forgot her/his question.
If the new question is not a duplicate, then the user who asked the new question should evidence in which points her/his question is not a duplicate. It can happen that different users ask similar questions that are not necessarily duplicates.
Alternatively, if the user who asked the new question read the old question, and s/he looking for a solution alternative to the ones already given to the old question, s/he should say that in her/his own question.