Hey there
It depends on a couple of factors, I'd say. Is this a forum where users have made the content, or is it a discussion/post that has covered a similar topic?
I get the feeling it's a variation of the latter, and if that's the case, I don't think you need to redirect it at all, namely for a couple of reasons:
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It's site content, and a variation of a similar topic, which can be seen as "related" to the theme of your other discussion (assuming that's how your site works).
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If the traffic isn't there in the first place, and hasn't taken off for some time - which includes no inbound links, external discussions and so on - then it's not going to be in the way of your other content.
I'd use the 301 if this is a duplicate issue, and you're worried that people won't find the real target of your campaign.
When you consider the themes for most sites, a lot of content is going to overlap and be very similar to other pieces. The Moz is a good example of that: so many blog posts with very similar themes; just variations of one another.
Hope this helps