What does it mean that Apple's double certification sent me another iPhone?

Apple's dual authentication will allow you to use a password for each device.

Double authentication is a new dual security authentication mechanism for AppleID in iOS9 system, and it is an upgraded version of the existing four-digit to six-digit two-step verification code. When you log in your AppleID on a new device or log in through a web page, the device you trust will automatically receive a six-digit verification code, which must be entered to complete the login.

Whenever you log in with an AppleID on a new device or browser, you need to use a password and a six-digit verification code to confirm your identity. You can get the verification code in many ways. You can use the verification code displayed on the trusted device to receive short messages or phone calls, or generate the verification code from the trusted device.