I have several users that are all using Outlook 2010 to communicate with our Exchange 2010 server. Over the past 2-3 days I've had numerous reports of individuals who have sent an email in the morning. The recipients received the email and then later in the day the email the sender sent appears in the sender's Inbox exactly like it would be if you took a draft message and dragged it into the Inbox - No From or Sent Date and when you double click it it opens exactly like a draft for editing.
I've been googling around for a few hours and can only find people that are syncing emails to MACs getting duplicate Calendar entries and people using HotMail/Outlook.com getting duplicates and deleted items returning etc.
These are all Windows XP w/ Outlook 2010 talking to an Exchange 2010 DAG with nothing syncing from any other sources - we do not permit outside email access or syncing with phones or other devices.
Any suggestions on how to isolate this?