"duplication" is percieved issue, nobody yet mentioned that in "real life" (tm) single installation of ant simply won't work. Remember, that ant picks up all the libraries in the $ANT_HOME/lib folder, and in order for build to work, lots of people add additional libraries there: junit, httpunit, xmluntil, anteater, etc, etc, etc. Now, imagine all the hurdles users will have to endure to make sure that they have all the necessary jars in all the necessary places.
Next step. Imagine that user uses more than one project on the same box with the same ant... And it requires some different version of some different third party ant add-on...
Huh? You lost me here. There is this little thing called a <path> element, and certain tags like javac and junit except a <classpath> that refers to the predefined <path> element. Guess what--you can include it from there... But maybe that's just my machine.
Anyhoo, since I am the odd man out, I will shut up now and let you guys get back to your regularly scheduled program....
--
"They that give up essential liberty to obtain a little temporary safety
deserve neither liberty nor safety."
- Benjamin Franklin