The Memoirs of Sherlock Holmes - The Stock-Broker's Clerk
Watson had moved out of Baker Street after he married Miss Morstan from the Sign of Four case.
One day, Holmes visits him with the client Mr Hall Pycroft. Mr. Pycroft tells his story:
He used to have a job at Coxon & Woodhouse but was laid off. After months of looking he finally found a job at Mawson & Williams, a stock-broking firm.
A man, Arthur Pinner visits him and offers him a much better job with his brother Harry Pinner. Pycroft visits Harry and discovers that they look exactly the same except for the hair color. Pycoft is given menial tasks and asked to report in occasionally. He notices that Pinner is only there in the completely empty office building except during the appointed time. He calls Holmes once he realizes that both brothers have the exact gold tooth filling with a peculiar mark on it.
Result: The Beddington brothers, famous forger and cracksman used Pycoft's new job and writing sample to impersonate him at the Mawson job. One kept Pycroft distracted during those days while the other one robbed the bank.