When this issue occurs, a user can resend you an email as an original sender unexpectedly, although the user is not assigned the "Send As" or "Send on Behalf of" permission. This issue occurs when the user is in online mode in Microsoft Outlook.
This issue occurs because of a designed logic.
To resolve this issue, install the following cumulative update:
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about cached Exchange mode and online mode, see Choose between Cached Exchange Mode and Online Mode for Outlook 2013.