Bug#629983: libc6 install aborts with "A non-dpkg owned copy of the C library was found in /lib."
I fell for this too: (had the /lib/ld.so left over from 2001)
It also took me too long to figure this out from the error message.
I recommend that the error message echoes out the exact path to the file it is having trouble with instead of a generic "... C library was found in /lib" message for everything it is searching for.
Otherwise, good work people. There are lots of reasons WHY I run Debian!