When you install watsapp on a new phone, it will ask for re-verification in the old mobile. I assume this is a security feature. 

I re-verified using another number put on the phone, but could still access other persons messages. Interestingly I could also send a watsapp message to myself.

So I am checking with watsapp whether this is a bug or an expected feature. 

Will update the response here.

Whatsapp's response was kind of funny. They did not even read my message fully, and sent below trouble shooting tips. When I was able to verify my number and even another number using whatsapp, they have provided steps to verify. My question is it is an expected behaviour to verify another person's whatsapp account with my number and then read his previous messages. Whatsapp's response attached below:

Sorry to hear you are having problems verifying.

1. Double check that you've entered the correct phone number.

   • Please enter your phone number in the full international format (+Country Code Phone Number).
   • See this article on how to format your number: http://www.whatsapp.com/faq/general/21016748

2. Make sure you have an active SIM card for the number you are trying to verify.

3. Double check that you have Internet service. If you are traveling abroad and roaming, keep in mind that service providers may have roaming restrictions and roaming can incur extra charges.

4. Check your SMS credits / payments

5. Send a test SMS from your phone to your number. When sending the SMS, enter your number in the international format (from step 1)

6. Reinstall WhatsApp

   • Delete WhatsApp
   • Reboot your phone (turn off the phone, take out battery if possible. Wait 30 seconds. Then put back the battery and turn your phone on)
   • Download a new version of WhatsApp: http://www.whatsapp.com/download/

If you are still unable to verify your account, please wait the designated amount of time to try to request another code and try voice verification. There is no way to expedite this.

We unfortunately cannot send your code through email due to security reasons.

If you still don't get your code, please ensure that your device is supported by checking this FAQ page

If you have not done so, please provide the following: 
1. Your phone number in this format: + CountryCode PhoneNumber 
2. Your device model

-- 
WhatsApp Support Team

I have sent a mail response clarifying my question. 

They clarified that it is expected and hence they ask customers to uninstall it before switching phones.