Hi,
your first question: making an analogy, an IP Address is like a personal ID. It is your PC's "ID". Each network interface in the same PC must have a different IP Address, so it's logical to say that one IP Address must be assigned to a specifical network interface. In one idea:One IP Address can't be the same to another.
your second question: ok if your OS is Win XP you can enter to cmd windows and type "ipconfig /all" and this command will show all your configuration or you can enter to your network connections and right click and select properties; there you will find protocols, and select TCP/IP then make a double click and there you will find your IP Address, DNS and Gateway (this method can be used in all windows platforms).If you're using linux/unix type in a console "ifconfig"
your third question: you can obtain the ipaddress via an email with the help of a sniffer in my opinion the ethereal is a good option. But it will only work right if your using it in the moment you send or receive the mail, so it can analize the packages.It will check the address of the sender such as the receiver.
I hope the information helps you.