Groupwise Global signatures are applied to every outgoing message. This includes replies and forwards that have already had the signature applied previously. We have an image and disclaimer message in our global signature. On long discussions we get multiple copies of the image and disclaimer embedded in emails.
Enable an option to check if the global signature has already been applied to an email - if already present do not reapply. a MIME header could be used for this purpose or even let the user specify a search string - if the string is found in the message do not apply the global signature.