Welcome to the BBF!
Are you using gmail as your outgoing email? That could be where the duplicate is coming from. HOW-TO: Prevent G-mail "Sent Items" from being sent to your handheld.
Your PIN is located at Options > Status, on your BB.
Each Blackberry has a unique PIN. This is not your 4-digit PIN for your SIM card, but an 8 character long hexadecimal value that is used to identify your device against the BlackBerry Enterprise Server. So this PIN is not used in the common understanding of "PIN" for logging in, like your using phone SIM card, ATM card, Online banking.
Sending private emails only between BlackBerry devices. A BlackBerry PIN is a special address code similar to a postal code or phone number, for sending emails only to other BlackBerry devices. This is not compatible with phone numbers or emails or SMS. It is an entirely different kind of an address. Not every BlackBerry user needs to use PIN Messaging, but this feature is there if you love it!