I've had my 8830 for two weeks, and it has done something similar 5 times already. I use G-mail, which I have integrated with my BB, and which i also download to Microsoft Entourage. It doesn't seem to be linked to whether or not I check my messages via Entourage. Just randomly, I will start getting every message that passed through G-mail since the last time it happened. today it was 200 messages!
I looked at BB's technical support page, and there is an article (Doc ID: KB11523) that states that the BB device downloads all messages with unique Message ID's. It goes on to say "If message IDs are being changed by the messaging server, duplicate email messages will be received."
The remedies provided are:
"1. Ensure that the message IDs of email messages in the integrated account do not change, or that email messages are not being added back to the inbox of the integrated account."
"2. Delete email messages from the BlackBerry device using the On Mailbox & Handheld setting."
Remedy 1 makes sense, but I'm not an IT administrator, and I don't know that you can control the inner workings of G-mail anyway. Remedy 2 makes some sense, unless (like me) you want to keep your messages in your G-mail and/or your Outlook/Entourage, but want them off your device.
I can't contact BlackBerry directly since I don't have a corporate account. Any corporate IT Admins or IT savvy folks out there who could shed some more light on this issue?