I see my friend on WhatsApp, but someone else is using this account.

Whenever a friend gives up a phone number, you should make sure to delete the number from your phone address book. Otherwise, you may incorrectly identify an account in WhatsApp as your friend's account, when the account in fact belongs to the new phone number's owner.

WhatsApp exclusively uses a phone number to identify an account and we display the names you have saved in your address book. The names you see in your WhatsApp Favorites / Select contact list, a specific chat, and in the Contact Info screen inside WhatsApp are not created within the application.

Cheers,
WhatsApp Support Team