Apart from anti-spam, you must have a software like Duplicate Email Remover to prevent duplicate e-mails from being added to the list and cluttering up your inbox.
Duplicate Email Remover is a plug-in for MS Outlook which allows to search for duplicate e-mails in the Outlook folders. The detected e-mails can be marked, labeled, removed, copied or moved to any Outlook folder. Moreover, it is also possible to apply filters based on time so that any e-mail received between the configured delays are processed and scanned automatically. You can even define the criteria for messages which must be put to the black list. Duplicate Email Remover is easy to install and entirely compatible with Outlook and Microsoft Exchange Server.