True duplicate detection is a science as any IT Database Manager can tell you.  Merging data from differently formatted lists compiled by people who type surnames in a street field or fail to field other aspects properly is a huge, costly headache.  We present for you two products that will give you the control you need to clean up messes that you inherited.  Both do a wonderful job and quite frankly their prices reflect their degree of capability.  Your purchase should be based on your needs.  One is perfect for the average company, the other, for those dealing with huge lists.