If a contact forwards an email received via nurture to multiple email ids, each time the recipients open the email, an open activity is recorded against the original contact in Nurture.
For example if [email protected] forwards an email received from Nurture to 30 people out of which 9 people open it, the activity stream of [email protected] would display 1 sent email activity and 10 open email activity (1 by self + 9 by the other recipients).