How to avoid duplicate contents

[Snowball1]

My forum has about 500 duplicate contents in google webmaster tools. 2 or 3 sometimes 4 different urls point to the same content despite the fact that they are different urls. how do I avoid duplicate contents because I learnt google will punish your site for duplication. Because of the duplicate content issue, I posted a thread here for help on how to redirect my index page to the forum homepage but I ended up getting infractions and the thread was deleted. Now I don't know what to do. Thanks