Envelope Manager’s Purge Duplicates feature works by comparing the 9-digit ZIP Codes of your addresses. Therefore you need to do a Batch ZIP+4 Lookup on all your addresses before using it. Because it’s based on 9-digit ZIP Codes, the Purge Duplicates feature works very well on widely-scattered mailings. It does not perform well on carrier route mailings, where you have multiple non-duplicate addresses with the same 9-digit ZIP Codes. On concentrated mailings like that, it probably will not identify all the duplicates.