I highlighted to a client recently that even though they had major on-page duplication issues they should first off look at their [Title] tags and meta data.
For me, when looking at duplication problems this is the first area you should review.
Many site designers and affiliates in particular simply print the same information in the these tags to save time.
In an ideal world you should have you most important keywords in the [title] tag but support them with used “call to actions” such as “Buy Sony MP3 Players” and then support them with a really useful meta description.
This tag should also include the keywords but support them with other searched on “call to action” keywords, maybe “For the Cheapest and best Sony MP3 Players try our online store.” If you can very these supporting keywords dynamically then go for it – you need to create as much variation as you can.
If the page you’re optimising is a category page then it would be advisable to list all the products, comma separated, in the Meta Keywords tag – just for completeness and because they can be used to help support the theme of the page as well as being useful for Yahoo! if you’ve included miss-spells.
Take time over the top half of your code – it’s the easiest place to be lazy, but where you’ll often get the most benefit.