Hi @mwilliams,
Well, the forwarded email is a relationship, right?
That is a potential explanation for your problem. In addition, the situation could also be caused by two visitors using the same device to complete a form, and possibly different devices on the same IP address (although that normally would not be enough to cause the issue.)
Short of turning off cookie tracking on your form, which would create a new contact for every submission, but would also cause you to lose all of your website tracking on actions before and after the form submisison, I don’t think there is a way to prevent this from happening.
You can build a workflow that helps you keep an eye out for situations like this so you can fix them before there is a problem. Just build your workflow to enroll every time the email address is known, AND ther first conversion date is more than 1 day ago. That should enroll existing contacts when their email changes. Also, be sure to allow them to be re-enrolled every time the criteria are met. From there, you can either send an email to your marketing team to check and make sure the change is valid, or add the contacts to a static list that you check periodically.