'require takes a filename and loads it if and only if it has not been required
before. 'require, like load, interprets paths relative to the current
directory.

Currently you can fool 'require by giving it a different name for the same file
(giving an absolute versus relative path, or prefixing with "./"). This is
considered a bug and may be fixed in future. Another possible future feature is
reloading a file if it has changed (as determined by its mtime, probably) since
it was last required.