Time and Time Again (Unabridged‪)‬

Henry Beam Piper was an American science fiction author who wrote many classic short stories and several novels during the "Golden Age" of science fiction in the USA.

In this story, a soldier of the future, wounded badly in battle during World War III, loses consciousness. When he opens his eyes again he finds that he is back in his boyhood past but with all the memories of his adult life intact!