What if I (Ubuntu packager):
a) don't like the Debian packager
b) can't convince the Debian packager that the update is important enough to warrant a package update
c) the update introduces a regression (which I consider minor, but he does not)
d) can't get a hold on the Debian packager, because he's away in Sri Lanka on a vacation
I believe 99% of the duplicate work done by distros is because of the "human factor". Check the mission statements/descriptions/history of most of the distributions. They start something like: "we were dissatisfied with the existing distributions" and continue "and we decided to make our own" instead of "we decided to join Debian/whatever".