It often happens you have 2 or more android devices (smartphone o tablet) with the same google account and you want to share apps between the devices or maybe you are going to change your mobile and you need to trasfer the apps from old mobile to new one.
Check all your apps
Take your smartphone (or tablet) you want to install apps on.
Open Google Play -> Settings -> My Apps -> All -> Select the app you want to install (for example Runtastic) and Install
Now tap on INSTALL and your app will be in both your smartphone or tablet, of course you don’t have to buy the app again.
Obviously share apps following this procedure works only between devices (smartphone or tablet) that are using the same google account, you can’t share apps between smartphone or tablet with different google account.