Why is WhatsApp showing your mobile number already registered?
It is common for mobile service providers to recycle mobile numbers. It is possible that the previous owner of your current phone number used WhatsApp and didn't delete his/her WhatsApp account. In that case, WhatsApp will show your mobile phone number in WhatsApp before you activate a new account. You may also see someone else's profile photo and About section attached to your phone number.
Why you should not worry?
If you are seeing your mobile number already registered on WhatsApp, there is no need to get worried. According to a WhatsApp FAQ page, this only means that the previous account wasn't deleted and therefore the old information still exists in the system. This does not mean that the previous owner of the phone number has any access to the WhatsApp account you activate with your new phone number. Your conversations and other WhatsApp data are secure, the page states.
WhatsApp monitors account inactivity to help eliminate confusion with recycled mobile phone numbers. In case an account is unused for 45 days and is reactivated on a different smartphone, WhatsApp considers it as a sign that the number has been recycled. After this, WhatsApp removes old data like profile photo and About section which is tied to the account linked with the mobile number.
