Duplicate content is bad for your website but the good news is that you can avoid having duplicate content. Keep in mind that duplicate content that can have an impact is your own duplicate content. What other websites will do with your content is way out of your control, but you have full control over your own content.
Some negative effects of duplicated content include the dilution of the anchor text, and fragmentation of your PageRank among other negatives. The only sure way you can tell that your content is duplicated is to make use of the ‘value’ aspect. Ask yourself if there is any extra value to your web page content. Do not reproduce content without any reason.
Is this page fresh content or a slight rewrite of the previous page? Make sure you put in unique valuable content. Your main concern should be not to send the search engines any bad signal as they are able to identify all duplicated content from very many signals. Just like ranking, notorious duplicated contented will be pinpointed, and marked.
In point one; you can have alternative ways in which your content is delivered. You have to select a default format and prohibit the search engines from recognizing the others, but still allowing the users to have access to the other formats. This you can do by adding the right code robot .txt file, and ensure you exclude all urls to the other versions on your sitemap also. Still on the urls point, use nofollow attribute on the website to eliminate duplicate pages as other people are able to link to them.
In point two; if you have one page that contains a rendering of an rss feed from another website and 10 other websites contain pages on the same feed, it could appear less of duplicate content to search engines. You will not risk duplication except if large parts of your website are based on the feeds.