Hi Mark,
If A,C, and E's page is similar to B,D, and F's page it is still consider dupllicate content. Based on Webmaster's definiton:
"Duplicate content generally refers to substantive blocks of content within or across domains that either completely match other content or are appreciably similar"
Each of your pages should be unique and different from other pages.
I suggest you to continue providing quality content and target the long tail keywords. That alone will help you generate more traffic. Furthermore, out ranking is not a problem. You should focus on getting to the frist page (providing quality content with long tail or regular keywords) and when you are on the first page, try to get searchers to click on your link using Title tag and Meta descriptions.
Out ranking just means they are ranked 4th and you are ranked 5th, 6th but as long as you have a better title tag and meta description. I believe searchers will click on the more attractive results.