It's actually a duplicate, but... Answers in the new question are much way better, than a duplicate. So, my reason was, why to close as a duplicate of question, that has some worse answers, than already given?
The general StackOverflow model is that multiple questions asking the same thing should be combined to one place where the answers can be found. This can get complicated when there are two or more questions with some good answers. Possible approaches include:
Duplicating the newer question to the older one (works best if this is spotted early and answers thus accumulate on the older question)
'Reverse' duping the older question to the newer one
Merging the questions (moves answers from one to another): best when both questions have good answers. This can only be done after duping.
Which route is best depends on the quality of the question (which is easier to read/find) and the answers.
In the case in point, the older question is probably better but the newer answers are perhaps stronger, so merging might be the best plan.