Are you sending from a gmail or outlook.com/hotmail/live account? Those services automatically copy sent messages to the Sent Mail or Sent folder, so you must uncheck 'Place a copy in' in the Copies & Folders section of Account Settings.

@sfhowes makes a good point: are you referring to sending two messages to recipient, or saving two messages to Sent folder? If you're referring to sending two messages to recipient, is the message being somehow orchestrated by your estimating software? More information will help. Thank you.