If you open the message. 'Resend' is on the Message Tab of the ribbon.
Alternatively you could use a macro to resent the message - see the code at
Macro to Resend Sent Message - however you will need to change the lines
Code:
' update fields if needed
olResendMsg.Subject = myItem.Subject & " (resend)"
olResendMsg.Send
to
Code:
' update fields if needed
olResendMsg.Display
to achieve the functionality you require.