How to prevent duplicate content issue when having same user comments in multiple content pages?

I have a question about duplicate content issue. I have pages with article, one page = one article. Below the ar开发者_如何学Pythonticle is discussion forum / comments box.

These articles have sometimes very similar subjects. So it usually happens that user comments / asks the same thing, which was already discussed in similar older article. But that's because the user doesn't know about the older article(s).

So for certain articles on the same subject I use one comment's box for multiple articles.

All the articles are my original content, however the second part of the page would be purposely duplicate content, because in this case - it's good for user ( that's what Google says webmaster should do - what's good for user ).

So my question is - should I fear this could be treated by search engines as duplicate content ? And if yes, what steps should I take to preserve this functionality and not to be penalized by Google and others ?

I don't think that you have to take care about this. Infact the duplicated content will be a little part of the whole content (which you say is brand new). Another fact you have to bear in mind is that also if the topic is the same it's very hard that people will write them in the same way.

Google covered this a few years ago:

The duplicate content "penalty" isn't to discourage webmasters from referencing the same content via more than one URL, but it is intended to prevent mass spamming of duplicate content from your (or worse, someone else's) site. You should be fine.