@regency-mho - Have you confirmed that the dups only (and always) happen when there is an issue sending the first email?
Does this happen if a single document is sent? The reason for this question is that the receiving email server might not like getting simultaneous multiple emails (addressed for the same user), from an outside domain. Might be some sort of spam detection.
Do the recipients in question use a third party mail filtering service like SkyScan Service?