Let's say you received an email sent through Emma and then used the forward option in your inbox to send the mailing to me; if I click a link in this forward, it's going to track as though you were the one that clicked it. That's because each recipient receives a unique copy of your mailings, with links tied specifically to their contact record. In this same scenario, if I clicked the link to opt out, I'd actually be opting you out! It's best to avoid forwarding for this reason.
There is an option in the footer of your mailing to share via email, which sends a fresh copy not associated to a specific contact. The clicks in shared mailings will not be tracked but you will be able to see how many times the emails was shared by heading to the Shares tab of the response page for that mailing.
Let me know if you have any further questions on this.