My Hotmail account will sporadically download a duplicate of an old message. This doesn't happen very consistently, but there are a few common characteristics, and I have a theory.
Every time a second copy of an old message is download, there is also a valid new message. In other words, if I don't have any new messages, I never get a duplicate of an old message. My theory is that the Hotmail plugin is checking messages in "pages". If a new message arrives in my Hotmail inbox while my plugin is in the process of checking for new messages, the resulting shift of all the messages in my account causes the plugin to think there are two copies of a message, and since only one exists locally, it downloads what it thinks is the "second copy". If my theory is correct, I hope that you're able to find a way to take that kind of situation into account. If it is unreasonably difficult, no big deal. This is just an annoyance. Thanks! Other relevant info: I typically receive 15-20 messages per day in hotmail. My inbox contains nearly 8000 read messages. (I don't leave any messages unread) My oldest messages date back to 2010.
