I checked one of your campaigns, and it does seem like the 301-redirect is working properly. I'm also not seeing any evidence of links to the "index.htm" version or other issues. I don't see evidence of both version sin Google's index. Not sure exactly what's going on here, but I'll run it by the support team. I don't think you have cause for concern.

I'm going to generally agree with (and thumb up) Mark, but a couple of additional comments:

(1) It really varies wildly. You can, with enough duplication, make your pages look thin enough to get filtered out. I don't think there's a fixed word-count or percentage, because it depends on the nature of the duplicate content, the non-duplicate content, the structure/code of the page, etc. Generally speaking, I would not add a long chunk of "Why Buy With Us" text - not only is it going to increase duplicate-content risks, but most people won't read it. Consider something short and punchy - maybe even an image or link that goes to a site with a full description. That way, most people will get the short message and people who are worried can get more details on a stand-alone page. You could even A/B test it - I suspect the long-form content may not be as powerful as you think.

(2) While duplicate content is not "penalized" in the traditional sense, the impact of it can approach penalty-like levels since the Panda updates.

(3) Definitely agreed with Mark that you have to watch both internal and external duplication. If you're a product reseller, for example, and you have a duplicate block in your own site AND you duplicate the manufacturer's product description, then you're at even more risk.