I'm not certain that this kind of duplicate content harms your SEO, but I can tell you what my company did to mitigate this kind of duplicate questions.
We merged threads When questions are exactly the same, we just merge them into one thread. There's no point in having multiple threads when the answer is going to be the same, and having multiple answers in one thread is much more helpful to whoever needs this information in the future
We pre-empted the question with a support or FAQ page
This ended up being a huge part of our help page strategy. When we noticed that people were asking the same question several times, we we would create a new support page with in-depth answers to solve that problem, and suggest that users read this related page before posting their question
We got rid of the forum... Right now, we don't have a public help forum (though this is something we'll likely open up again in the future, when we have more eyes to dedicate to monitoring the forum!), so what we ended up with is a bunch of in-depth help articles, as well as close monitoring of which popular search strings are not turning up good results, and which questions are popping up very frequently in our support queue.
I hope someone else can come along and answer the duplicate content part of your question, because I would love to know as well!