The reasons for duplicated messages appearing in Outlook mailbox vary. They can be caused by incorrect functioning or configuration of the main synchronization software; problems with mail server or mail client (for example, if the message copies are left on the server and you have to re-create an account in Outlook); or just by human error when doing a copy-paste of a number of selected messages; etc. Any of these events can leave you with numerous duplicates and you will want to eliminate them. This task is easy, just take Duplicate Email Remover and rid all your mail folders from duplicated messages and posts even if the source message and duplicate are located in different folders.
