Hi, amee!
Your post about Bach's Double Violin Concerto caught my attention.
I don't know just how each of Bach's concertos originated, but I do know that Bach would often recycle his own or other composers' material by transfering music from one medim to another. This, I think, was a common practice during the Baroque era and throughout the history of western music for that matter.
I would be interested to know more about your friend's performance of the piece. Was this a performance with orchestra, a reduced version (e. g., violins and piano), or what?
I have a recording of this piece but I've never heard it performed live. After what you've had to say about it I can't wait to hear it without a stereo system in the way!
Regards,
Thomas