Few pages of my website have duplicate content, does it affect in SEO

Yes. Google dislike content those are copied from within a same website pages or others website pages.

"Don't create multiple pages, subdomains, or domains with substantially duplicate content."

Google prefers to show unique content to its users when they do searches. If user come across same content multiple times from various websites, user annoys. In order to give best quality results, Google prefers to show unique content to its users.

If your website has duplicate content, then your website should add/edit the content in a way that it should be keywords rich and unique.

Reference:

http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=35769#3
http://www.sitemaps.org/

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