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.
- comment-pln: Comment macros for use in Plain TeX
- automagic: Automagic numbering in Plain TeX
- mpostinl: Embed METAPOST figures within LaTeX documents
- letter-plain: Letter macros for Plain TeX