So I read that you can attach a duplicate to any card it is the duplicate of and then discard or kill it when that character is to be killed or discarded. This makes a duplicate just like a "Bodyguard" card, right?

Generally speaking, yes, you could look at it in such a way in terms of effects when you are starting to play the game.

But really there are a lot of differences.

The duplicate also has additional utility in that it can save from other effects like return to hand, return to deck or return to shadows.

The duplicate is also not an attachment and cards that interact with attachments will not effect a duplicate.

The duplicate has no condition for play other then it, in most cases, must have the same title as the card it is duplicating.

The bodyguard has to go on a character, who must be a lord or lady, and if those traits are removed for some reason then the attachment would no longer be valid and would be discarded.

There are undoubtedly some other things that i haven't mentioned but they are, i think, the major points.

Duplicates save all instances of a character (or card) leaving play - i.e. also for saves from Coldhands, Pulled Under ...

Duplicates give the card that it is a duplicate the ability to do this as a "Response" - so that can be cancelled with eg Iron Throne

A duplicate is, per the FAQ, a card with no text, titles, traits, or crests.

A duplicate gives the card it is a duplicate of the ability to save it from leaving play as a "Response" effect by discarding it from play. Keep in mind that you are not killing the duplicate, you are discarding it. This is an important distinction because you need to put it in the discard pile.

So, you are absolutely correct that it's like a "body guard".

(But, as has been mentioned, a Bodyguard on steroids, because dupes can do more than Bodyguards.)

Duplicates save all instances of a character (or card) leaving play - i.e. also for saves from Coldhands, Pulled Under ...

Duplicates give the card that it is a duplicate the ability to do this as a "Response" - so that can be cancelled with eg Iron Throne

Just to clarify duplicates wont save against pulled under as it is an end of phase timing and there are no response opportunities per the faq on end of phase resolution.

Thanks for that clarifiation re Pulled Under - that makes it a bit more useful