First of Google does not "penalize" you for duplicate content, unless you're doing it on a massive scale (Panda). If Google detects duplicate content on your site, it will only display one version of that content in the SERPs. Still not ideal, not quite a penalty.
Perhaps more importantly, why do you have multiple landing pages that differ by only 10 words? Aside from the duplicate content issue, how are you "optimizing" each page for different keywords? If you are just changing the title and the URL, then it's probably not worth it from an SEO or user perspective.
If you want to rank for multiple keywords, write rich content which is relevant for multiple keywords or create multiple pages which substantially different and specifically aimed at your target keywords. Changing 10-15 words isn't optimizing for anything.