Something like IdeaTorrent or the bug reporter on launchpad.net. When adding a new idea: 1) You type a few key words (or the title) for the idea. 2) DialogueApp lists ideas which rank highly for the keywords (or some of the keywords). 3) If your idea is similar enough to the ones listed, you can contribute to that idea instead of creating a duplicate. 4) If you can't find a similar idea, there is the option to create a new one.

Less time would be wasted writing duplicate ideas. Less moderating time would be wasted looking for and marking duplicate ideas. Also there would be no need to tell people to search for their idea before adding it (which people ignore anyway).