Al Iverson's Spam Resource: Receiving Duplicate List Messages?
“The other day, somebody asked me what causes a recipient to receive the same message more than once. I run into duplicate message issues perhaps once or twice a year; not too often, but often enough that a recipient gets really angry at the sending ESP, assuming it’s they’re fault, because it doesn’t seem to be happening with other email.
To that end, I’ve compiled a few tips I think you might find useful when troubleshooting a recipient receiving multiple messages.”


