If your form fields are pre-pouplating with another person's information who has never submitted a form on your browser, it is likely due to a forwarded email.
HubSpot forms pre-populate with known fields by default. This is done by associating the HubSpot tracking cookie in your web browser with your existing HubSpot contact. Based on how emails are tracked in HubSpot, forwarding an email will associate the second contact with the original email recipient's tracking cookie. This will result in the second contact's information being associated with the original contact's browser. This will lead to any forms that are set to pre-populate with known values to show the second contact's information on your browser. This is also why any email clicks from a contact who was forwarded an email are attributed to the original recipient.
If you or your contacts are noticing another person's information pre-populating your form fields, you can clear your browser cookies and then re-submit a form to become associated with a new tracking cookie.