Spiders do not like on same site, the same content repeated.
If you sometime have use big sites like stackoverflow etc, you can see and note how they take care about duplicated questions or contents.
Spiders do not like this. It is implicit in the logic of the code of a crawler.
Imagine: i could repeat same content several times with easy and fill a big site with thousand of pages, with content that maybe differ little one from other.
So, it is not convenient for the site, to have something like this. We could also pass the content of the entire post, as hidden content on page, that the spider will see and index. But after he will found same content in other page/position of same site. This is really bad.
You are speacking only about tags instead .... that can be a very good solution.
Yeah i've put on list this one from long time ... something will be done about ... and some little tip more
By the way as you can see here, to me it is sufficient to have maybe on full forum, a link to iframe for those are coming first time (i will prepare something nice about, just a little js code) on site maybe after a google click that point to full forum. email links already point to iframe, if needed.
Good SeO results are coming from just this main principles: external sites that are linking you, and related content about that maybe match the same arguments of your site. So direct clicks into results, after a search into yahoo,google etc
They use also much more data to calculate the rank of the site for an argument, based on various services they offer, we can imagine (and we hope that they not push who they want in top, discarding someone else). But all lead to the same: content, related clicks, external links your site point to mainly, and links that point to you from external.