(I could use some help with tagging.)
There are many seforim out there that are the compiled letters or teshuvos of this or that Rabbi. Those that come to mind are "Igros Moshe", from R' Moshe Feinstein, and "Igros Kodesh" from the Lubavitcher Rebbeim.
My question is, since all of those letters are responses to people, they were presumably mailed out to their intended recipients. So where did/do the compilers get them? Do they go around to all the people who have letters and ask them for copies? Were the originals typed in triplicate?
I know that Rabbi Dr. Nissan Mindel, who was one of the Lubavitcher Rebbe's secretaries (shamoshim), had a collection of (English) letters that (I assume) he typed out for the Rebbe. (Edit - After consulting with the current owner of those letters, I was told that @msh210 was correct. Rabbi Dr. Mindel would take dictation in shorthand and then type up the letters in duplicate.)
Did these Rabbis keep personal copies of all their responses?