In current environment, there is nothing that can not be duplicated.
Its a question of cost of making something difficult to duplicate or the cost of duplicating something that has been made harder to copy.
So, whether you use screen printed letterheads or you use computer generated imaging on your letters, it can be very easily scanned and copied. Today, even a id size / passport size photos are being xeroxed (photocopied) instead of taking a fresh photo print.
The only thing "most" common scanners cant copy is water marks. But that also is being factored into in the latest scanners.
So the question comes is this : What do you want it for ? and what are you scared of happening ?
Remember also that an outsider will not know what safety features you put in your letter head and therefore he wont know whether the letterhead is fake or real. So again, what are you trying to solve, the answer depends on that
From India, Mumbai