The idea that a copy should be produced in workshop is hardly a surprise. In our book on the Madonna and the Yarnwinder,  Thereza Wells and I showed that the two prime versions developed alongside each other, in this instance with Leonardo's participation in both. The only surprise is that a copy should be made of an intimate, domestic portrait of a bourgeois sitter. Perhaps Francesco del Giocondo wanted two versions. But it is odd. The implications of the landscape for the dating of the Mona Lisa - the background in the copy is aligned with drawings dateable to after 1510 - provides useful confirmation that the painting took a long time, but is not surprising. Was it ever completely finished? Were any of his paintings completely finished? The London Virgin of the Rocks, which was supplied for the frame in S. Francesco in Milan, is not finished. Only the Louvre seemed to think that the ML was completed before Leonardo left Florence in 1507.

Perhaps I shouldn't complain. It all helps sustain interest and helps sell (my) books.

By the way, we have absolutely no reliable evidence about what Salai looked like - and almost no firm evidence of how he painted. The pretty boy with ringlets, often identified as Salai, was a favourite facial type for Leonardo well before Salai came on to the scene.