Two different IP addresses is extremely strange... how about User-Agents (check the details of the Filled Out Form activity)?
Have you run a reverse DNS lookup on the IPs?
PIease have a look at the activity log.If there are a lot of web page accesses which are visiting the same page.One of the possible reasons will be that this email was forwarded to the other coworkers,if other coworkers click the links in marketo emails,there activities will be tracked as the same user.
The Visit Web Page would be tracked as the original recipient, true. But unless the form is PreFilled and not changed/reset (or Known Visitor HTML is used) that wouldn't make Filled Out Form activities appear under the same recipient. Those would be under new or merged leads.