This question has been asked by so many users so here is the proper Tutorial.

1. Download your desired IPA and import in "App Library"

2. Go to "Apps" section then "Unsigned" section, click on your application that you want to duplicate and click "Signature"

3. (IMPORTANT): Click "Bundle Identifier" and in the popup, add any text along the current mentioned bundle id and click "OK"; eg, net.whatsapp.WhatsApp to net.whatsapp.WhatsApp2

4. Install the app. Enjoy!