maybeload – Prevent reloading of files
Defines a command \thisfileis, which will become \endinput if it's called for a second or subsequent time with a particular file name.
Maybe you are interested in the following packages as well.
- automagic: Automagic numbering in Plain TeX
- letter-plain: Letter macros for Plain TeX
- list: List ASCII text files
- label-pln: Print address labels in three columns