A Gathering of Widowmakers (The Widowmaker #4)

Jefferson Nighthawk, the original Widowmaker, is married and retired, looking forward to a peaceul old age. But when his two clones try to kill each other, he realizes that he can't turn his back on the situation -- or the fact that there is an alien menace out there that will require the combined might of all three Widowmakers, if even that is enough.