I started a website about a month ago then quickly changed the url, but Google still has both listed, so I think the new one might be getting hurt by duplicate content. How do I alert Google about the duplicate content, so they can get rid of it from their rankings?
One way would be to remove the old pages from the server. Another way is to put the following code into the head of each page you want to remove from the index: <meta name="robots" content="noindex">
This link should help you find what you need: http://www.google.com/support/webmasters/bin/answer.py?answer=35301
301 redirect or Google's webmaster tools... there's a built in feature to remove content from the index.