I believe it was Loren D. Estleman who said somewhere that if Dr. Watson had written all the tales that have been posthumously assigned to him he would never have had time to actually join Sherlock Holmes on his cases. Here are some volumes of such collected efforts. There is no telling how many repetitions of tales are in these collections.
The Final Adventures of Sherlock Holmes...Collected by Peter Haining*
The Lost Adventures of Sherlock Holmes...Ken Greenwood**
The Further Adventures of Sherlock Holmes...Collected by Roger Lancelyn Green
The Exploits of Sherlock Holmes...Adrian Conan Doyle and John Dickson Carr
The Confidential Casebook of Sherlock Holmes...Edited by Marvin Kaye
The Game Is Afoot...Edited by Marvin Kaye
The Secret Cases of Sherlock Holmes...Donald Thomas
My Sherlock Holmes...Edited by Michael Kurland
The New Adventures of Sherlock Holmes...Edited by Martin Harry Greenburgh & Carol-Lynn Rossel Waugh
*Three different copies of this.
**Two copies of this; the stories are based on radio scripts by Denis Green and Anthony Boucher, featuring the talents of Basil Rathbone and Nigel Bruce.