Hey guys…

I want to make a suggestion, which will be a duplicate of a really old topic, and if I post on it I will be necroposting, and if I make a separate topic it will be closed as a duplicate.

What should I do?

Either is fine. Because you are contributing to the topic, it's not considered necroposting. But because the other one is old and not used, it's not really considered a duplicate. Necroposting in the Suggestions forum would be something like saying “Bananas” or something else that doesn't contribute, or contributing, but on a feature that already exists.

Necroposting is the right thing to do in this case; it's not really necroposting.

I had this question before and I got a pretty unanimous answer.