Duplicate Messages delivered when sent to a group

I have a user’s name in a distribution group in the Domino Directory. When I (or anyone) send a message to that group, the user receives two messages.

The messages are identical, except this: In the second message delivered, these fields are populated:

$AssistMail = 1

RouteServers = [the same Domino server, but listed twice]

What might be causing the second message’s delivery?

Subject: an agent or rule

$AssistMail is added when an agent or rule produces the document, this prevents the agent or rule from looping.

Subject: Seems to be resolved…

Edited the ACL of the user’s mail file. For some reason, he was Person–>Manager, but didn’t have delete authority. He does now, and messages are not being duplicated.