CSK and Rajasthan can retain a player who played for Rising Pune Supergiants and Gujarat Lions. In the case of Ben Stokes, he did not play for any other team before he played for Rising Pune Supergiants. So technically, CSK and RR have a right to retain him. If any other team bids the highest for Ben Stokes, CSK and RR can match that highest bid. Then between CSK and RR, who matches the higher of the highest bid can retain Ben Stokes. I believe this is how it might happen, although the exact details is unclear.