We have noticed in the Activity History of some records that incoming email is being duplicated, sometimes many times. Checking the headers of the email indicates that the Email records are being created by the same original email.
This behavior is usually to be due to the configuration of the email listener (Inbox).
Review the Field Separator Value - if it is configured as a semicolon, which is commonly used in email headers to separate a list of addresses, then the application will create a separate record each time it hits one of these characters in the incoming email. So in the case of a threaded email forwarded to the listener, it will create many separate Email records, one for each email address in the thread.
I would recommend changing this to something less common such as a pipe | character.
Open the Inbox workspace (More... -> Inbox)
Open the affected inbox
Change the Field Value Separator entry from a semicolon (;) to a pipe character (|)