This is a feature of many FOSS projects, sadly, and I don't really get what aspect of human nature drives it, but it must be something.
Not counting efforts where someone forks an old version because they don't like where the team are taking the new version (Trinity vs KDE, MATE vs GNOME)...
There are 2 current forks of Mandriva doing much the same: Mageia and OpenMandriva. (There are others but with deeper differences.)
Claws is a fork of Sylpheed, both continuing separately.
Cinnamon, the Zorin OS desktop, and GNOME Flashback are all doing more or less the same things with the same codebase.