If your form fields are pre-popuplating 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 a users web browser with their existing HubSpot contact. Because of how emails are tracked in HubSpot, when a contact forwards an email to a second contact, the second contact becomes associated with the original email recipient's tracking cookie. This will associate the second contact's information with the original contact's browser. This leads to any forms that are set to pre-populate with known values to show the second contact's information on the first contact's browser.
This same functionality 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.