Subscribe by Email widget using IE9 "You are already subscribed!!"

The plugin works fine using FireFox, however when using IE9 when the user tries to signup using the wisget they receive the message "You are already subscribed!!" and the subscription isn't created. (When a new subscription is created via the admin page it is created fine and notifications are sent out.)

I've seen some reference in other topics about this plugin to javascript errors, which might explain why the problem is only happening in IE, but I don't see any error warnings.

Thanks
Jack