Whenever You are connected to the Internet always you have two ip addresses. your public ip and your private ip. Your IP address has to be unique for you to access Internet.
For example, if you have multiple computers within your home you may want to use private IP addresses to address each computer within your home. In this scenario, your router get the public IP address, and each of the computers, tablets and smartphones connected to your router get a private IP address from your router via DHCP protocol.
So Your private ip address must be unique within your router connection. but it can be the same as outside one. But your public ip always be unique all over the world.