sign and encrypt

[mkontakt at gmail.com]

I have seen in the spec rfc3156 that a message should be signed and
then encrypted, but hypothetically if send a message to someone I do
not like and sign it and then encrypt it he/she can forward it to
someone else pretending that the message was originally from my self.
Is there anything I have missed in spec or in gnupg to forbid this?

Thank in advance
Martin