You may encounter an issue whereby a user switches on the Out of Office in Microsoft Outlook and 2 messages are being sent, 1 an older out of office and 2 the newer one just set-up.
This issue can be caused from a variety of reasons, sometimes if a migration has had issues (then these may not have been noticed at the time) sometimes this can be caused when Outlook Web Access has been used to set-up up the Out of Office.
This can however be cleared by removing all server and client based rules, by simply launching Outlook from the Run Dialogue and using the /cleanrules switch.
Choose Start > Run and type outlook.exe /clean rules
Once you have opened Outlook, switch off the Out of Office, Exit Outlook and switch this back on.
You should now find that the issue has cleared.