AppleScript, Mail.app - Forward command doubles content - Mountain Lion

Question

I'm trying to write an AppleScript to Forward or Redirect an email. It works fine but doubles up the content when I change any of the new emails details. Eg the subject, 'to recipient', etc.
My code is:
tell application "Mail"
          set theSelection to selection
          set theForwardedMessage to forward (item 1 of theSelection) with opening window
          tell theForwardedMessage
                         set subject to "{plain} " & subject
          end tell
end tell
This is under Mountain Lion, 10.8.2, Mail.app is version 6.2 (1499).
Any help would be much appreciated.
TIA, James.

Answer 1

Thanks Pierre. That's really helpful.
On its own that didn't work, but if I put in a 'display dialog' command it does. Ie this works:
tell application "Mail"
          set theSelection to selection
          set theForwardedMessage to forward (item 1 of theSelection) with opening window
          set theSubject to subject of theForwardedMessage
  activate
          tell application "System Events" to tell process "Mail"
  display dialog "1"
                    tell text field "Subject:" of window 1
                              set focused to true
                              set value to "{plain} " & theSubject
                    end tell
          end tell
end tell
I also want the recipient filled in, but it doesn't work for me. In this code it displays the 'A' dialog then stops:
tell application "Mail"
          set theSelection to selection
          set theForwardedMessage to forward (item 1 of theSelection) with opening window
          set theSubject to subject of theForwardedMessage
  activate
          tell application "System Events" to tell process "Mail"
  display dialog "1"
                    tell text field "Subject:" of window 1
                              set focused to true
                              set value to "{plain} " & theSubject
                    end tell
                    tell text field "To:" of window 1
  display dialog "A"
                              set focused to true
  display dialog "B"
                              set value to "[email protected]"
                    end tell
          end tell
end tell
Ie it never gets to the 'B' dialog.
So perhaps that field is called something else? How do you find out what the 'To:' field is called?
Thanks,
James.