The Duplicate
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The Duplicate is a 1988 William Sleator book about a boy named David who finds a device called the Spee-Dee-Dupe. The Spee-Dee-Dupe will duplicate any living organism that presses down on its lever. After testing the device on his fish, he becomes emboldened and tries it on himself. He soon finds himself in conflict with the duplicate, as each believes himself to be the authentic David.