Do you mean that the form fields are pre-populating? This isn’t something WordPress normally does, and would likely be the browser filling it out based on the user’s address book or previous submissions.
Or do you mean that it is appearing as if they are logged in as another user?
If so, are you using any type of caching plugin (WP Super Cache, WP Rocket, W3 Total Cache etc.)? These plugins should have a setting to disable for logged in users. This way it won’t cache versions of pages that display a logged in user’s details.