Security Explorer 8.x - The Parameter is incorrect when enumerating mailboxes (4262793)

The information in this article applies to:

Security Explorer 8.x 

Title

Error enumerating mailboxes “Item has already been added. Key in dictionary:”

Details: “The parameter is incorrect”. 

Problem Description 

When mailbox storages are being enumerated within the Exchange Security module the error message “Item has already been added. Key in dictionary:” Details: “The parameter is incorrect” is displayed:

Cause

This error will occur if there are duplicate user names in Active Directory, even though the Alias and e-mail addresses are different. The native tool will allow these user accounts to be created if they are residing in different Organizational Units (OUs). 

Resolution

Change the user accounts so that they no longer have matching first and last names.