When running genhdlist I made a mistake putting the same RPM on both instalation CDs. It gave no error on this. During the instalation anaconda asked for a disk numbered -2167455... (don't remember the exact number was about -2x10^31).
I think you should improve genhdlist to warn about duplicated files and anaconda to do some sanity checkings before asking for something like that...
Added a nice error message.
Verified that the error message came up.