You can start your letter as follows -
"In supersession of our dt.------(mention the ref. of previous letter), we hereby state that Mr...............
Just retype the contents of the same(previous) letter as it is, without changing anything, sign it and affix a rubber stamp mentioning "DUPLICATE" at the top of this letter.
Retype the contents of previous letter and change the date & reissue with current date.
You may decide on any of the above or other options.