Man With Four Lives

Novel with rather a weird premise: during World War I, while serving in France with the Canadian Army (as did the author), the protagonist of this tale has occasion to apparently kill the same German officer twice, and then to encounter him yet a third time, and witness his execution as a spy -- all of which has the cumulative effect of driving him stark raving mad. He eventually recovers, but I know you've only counted to three, and sure enough, years later when he and his wife are skiing in Sw...