duplicatesuppression -- why?

[Patrick Goetz]

I've been wondering about this for a while, given that there is an 
entire db file devoted to this task, indicating a considerable 
investment of resources.

 From the imapd.conf man page:
-----------------------
duplicatesuppression: 1
If enabled, lmtpd will suppress delivery of a message to a mailbox if a 
message with the same message-id (or resent-message-id) is recorded as 
having already been delivered to the mailbox. Records the mailbox and 
message-id/resent-message-id of all successful deliveries.
-----------------------

How often does it happen that the same message is being delivered twice? 
  I can't think of a single SMTP scenario where this would be a major 
concern, but then I'm not an expert.

Is this another crazy anachronism (like all the NNTP features) that I 
should just turn off and forget about once and for all?