Webpages easily contain a lot of duplicate content. This however, can hurt your Google pagerank. Duplicate content can also slow down your pages.
Currently I’m building a new site with some friends. This site has a sign-up form with all the usual fields like name, email, country and so on. Now, to make sure the new user enters a correct country, they are presented with a complete list of countries in the world. This list was, until a few days ago, loaded into all the pages on the site = bad for performance!
This is how I solved it: