For my site Post My Greetings, we have 2 URLs, one with .com and another with /default.aspx and both pointing to the same page. Would there be any duplicate content issue in this case? Should I use 301 redirection?
Guessed it right! You should. Same content for more than one URL is a sure sign of content duplication, and you have exposed your website to the risk of being filtered out. So act ASAP. And If I am not wrong you also have more URL issues. Check out the home page URL with and without www PLUS with and without /default.aspx. You have done really good if you do not have www issues. Contact your hosting service provider for the 301 redirection, if you are running your site on an MS IIS server. 301 Permanent redirection is highly recommended, however. All The Best
Yes, When a single page can be accessed by multiple URLs, (e.g. with www, non-www, /default.aspx and so on ) search engine assumes that there are multiple unique pages, sharing same content and search engines are very strict about duplicate content issues and will immediately panelized that site, so prevent dilution of page relevance use 301 redirection.
It is nice that your sites are still alive. And before being penalized you asked for help. However, change that as quick as possible.