If you have installed Eclipse 3.3 from an official bundle or you have installed the Mylyn plug-in then there's a good chance that you might have started seeing duplicate entries when using content assist.

When the Mylyn plug-in is installed, it registers itself as a provider of content assist data. The first suggestion comes from the default content assist provider in Eclipse and the second one is generated by Mylyn.

Open the Eclipse preferences screen. Navigate to Java -> Editor -> Content Assist -> Advanced.

[img_assist|nid=27|desc=|link=none|width=424|height=199]

Uncheck the duplicate entries that do not contain "(Mylyn)" in their name. Problem solved.
If you're not making use of Mylyn features, you will receive the same suggestions as from the default suggestion provider.