This issue has been reported by users on various Android support forums:

http://www.droidforums.net/forum/droid-2-tech-issues/158258-droid-2-sending-multiple-emails.html

  •  Seems to affect Motorola devices on Exchange 2010

  •  Attachments also seems to be a cause

  •  Possibly has to do with original message not yet fully downloaded before Forwarded or Replied to

Resolutions:

  •  The manufacturer and mobile providers recommend disabling SMARTFORWARDING (aka Auto Download Attachment) on the device. Some posts indicate that this doesn't fully resolve the problem

  •  Install a 3rd party email app like Touchdown. This appears to be the best fix.