why duplicated manual pages?

["Jeremy C. Reed" <reed%reedmedia.net@localhost>]

I see we have a variety of duplicated manual pages. For example:

src/usr.sbin/mdsetimage/mdsetimage.8 not used
src/gnu/usr.sbin/mdsetimage/mdsetimage.8 is used

src/sbin/modstat/modstat.8 not used
src/usr.bin/modstat/modstat.8 is used

src/dist/am-utils/wire-test/wire-test.8 not used
src/external/bsd/am-utils/dist/wire-test/wire-test.8 is used

src/dist/am-utils/fsinfo/fsinfo not used
src/external/bsd/am-utils/dist/fsinfo/fsinfo.8 is used

And many others.

What is the purpose of keeping unused manual pages in the source tree? I 
guess I can understand the dist and external but not both. But I don't 
understand unused duplicate man pages under src/sbin and src/usr.sbin.

  Jeremy C. Reed