We started to doubt a customer’s sanity when they said that they were seeing name and address information already populated in their WooCommerce checkout. Even weirder, it was contact information for somebody else.

Come to find out that their default hosting options had aggressive caching running and the site was caching every page, including these 3 pages that you NEVER want to cache:

/cart/
/checkout/
/wp-admin/
/my-account/

Obviously, you need to make sure that these are the URLs that your installation of WooCommerce is using, as these are the defaults that we use.

This was a consulting project, so we were not involved with the hosting setup. We host with Paranoid Hosting and, by default, those cache rules are already in place.

Source: https://wordpress.org/support/topic/when-guests-checkout-checkout-forms-are-pre-filled-dont-know-how-to-remove/