A question where every coder/DBA/QA and Tester should now about
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In short:
1. Select the duplicate key values into a holding table
2. Select the duplicate rows into a holding table, eliminating duplicates in the process
3. Delete the duplicate rows from the original table
4. Put the unique rows back in the original table
Here is step-by-step answer from Microsoft
http://support.microsoft.com/kb/139444