This will likely be a multi-part story because it turns out the solution was one of the more obscure things I think I’ve run into AND I thought it was worth “dumping” everything I found out in the process here for my own reference and anyone else wandering through. I have a customer that uses Microsoft Outlook 2003 and has selected to “leave messages on server”, then delete them when they’re deleted from the recycle bin. This normally works well, although I do recall one (or two) instances in the past when he got duplicates.