What might cause a user to receive duplicate emails?

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http://serverfault.com – One of our customers is receiving duplicate emails from our application, even though as far as I can tell from the application and SMTP logs, we're only sending it out once. What are some of the ways that an email might be duplicated between sending it out and receiving it? Some more details: the emails are being sent via IIS6. The duplicates that the user receives really are exact duplicates, right down the message headers (including the same Message-ID). There's no apparent duplication in the SMTP source logs - I'm not a sys admin myself so I don't have much experience with these, but it s (HowTos)