The problems with NOT implementing 301 redirects is really not that of a duplicate content issue. It has more to do with split page rank/link juice. For example say your existing mydomain.com/page.php?id=xxx page has 100 inbound links from external sites and 100 inbound links from inter pages on your own site. It's likley those 200 inbound links are the primary reason that the old ugly URL is "currently" ranking well. And then you throw up mydomain.com/page-title-some-keyword.htm with the exact same content as the old page. First of all, you will more than likely want to change all of your links on YOUR site to point to the new SEO friendly URL which is going to reduce the number of inbound links to your old URL from 200 to 100. BAM! It's rankings just dropped. Your new URL has 100 inbound links but they are all from internal pages so it's not going to rank well either. Say, over the upcoming years, other sites ll begin linking to your new SEO friendly URL, and you pickup 100 links. You'd think you might be back where you started as far as ranking (because the new URL now has 100 inbounds from external + 100 inbounds from internal). But other sites have been getting links during this time also, so those same keyword phrases are now more competitive... harder to rank for. The purpose of the 301 redirect is to transfer credit for all of the old URL's inbound links over to the new URL. Once the 301 redirects are in place and after the search engines have recrawled all the inbound links to the old URL, discovered the 301 for each inbound to the old URL, and transfered credit for the inbound links to the old URL over to the new SEO friendly URL, it will be as if the NEW URL has 200 inbound links (it has gotten credit for all 200 inbound links to the old URL). Actually you'd still want to change all links on your own site to point directly to the new URL (eliminate the 301 where you have control of the link). ANYTIME you rename/move a page, ALWAYS 301 redirect it to the new location. Otherwise, you're starting from scratch trying to make the new page rank all over again. You've thrown away the work you did building links to the old page.