The Double

"All that he had dreaded and guessed at had now taken place in reality. He gasped for breath, his head went round. There was the stranger, sitting before him on his own bed, also wearing a hat and coat, faintly smiling"
A lonely government clerk - shy, awkward, blundering - finds himself pursued by a mysterious stranger. Somehow he looks familiar. In fact, he realizes, he looks exactly like him. He even has the same name. But, unlike him, he is charming and confident.
Soon the stranger starts insinuating himself into his life. He works at his office, stays at his apartment, ingratiates himself with his colleagues. No one seems surprised.
Who is he? What does he want? Is he a double, or something darker altogether?