E.g. you have the app "Viber". Now you want to register 3 different Viber accounts (for 3 different phone numbers respectively). In this case, I want to duplicate the Viber app to Viber 1, Viber 2, Viber 3 etc. So that they each maintain different documents and hence maintain different phone numbers.
I think what you are asking is how to install several ipa's of your app onto your device side by side. For this, you build your app with a different bundleID each time you want to install it new without replacing the previous build. Note that you have to have proper codesigning profiles for each bundleID (as from iOS' and code-signing perspective its an entirely different app).