Backup the db before you try this. You'll also need to add a unique ID column if there's not one already.
DELETE
FROM MyTable
WHERE ID NOT IN
(
SELECT MAX(ID)
FROM MyTable
GROUP BY DuplicateColumn1, DuplicateColumn2, DuplicateColumn3)
http://blog.sqlauthority.com/2007/03/01/sql-server-delete-duplicate-records-rows/