Duplicate Content Issues Caused By URL Parameters

If there is more than one URL that you can use to display the EXACT same page on your website, search engines will consider that you have duplicate content.  This can negatively impact you.  If there are 2 different URLs that bring the same page up, there is a good chance when people link to your site, they will use either of the 2 different URLs, which shares the number of incoming links across the 2 URLs instead of just 1 URL.  It's like having each way bets, instead of betting on the winner.  The more links you can get to your site, the better chance you will get a better Search Engine Position (there are lots of other factors)

So, if you have dynamically created pages, test them out to see if there is more than 1 URL that you can use to display the exact same page.  If there is, work out how to consolidate it down to one page.

ALSO, in the process of merging your URLs, don't loose your existing search engine index link love - no matter how poor it is.  Use 301 redirects to forward the dupe URL to the correct URL.  Search engines will see this, and adjust themselves accordingly.
This tip doesn't help you get a better search engine position, it just helps prevent you getting a worse position than you would.

You want a better search engine position?  Just focus on giving your users a good experience on your site.  The search engines will take care of the rest.  Read that sentence again.  It is way more important than you think.