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					Originally Posted by CT1  This is not an illegal substitution, since CRs must be eligible substitutes.  It's an unreported substitution.  The CR is now in the lineup, and the P or C has been out of the game once and can reenter.
 However, this can't take place in the same half-inning.
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 2-36-3g "An illegal substitute is a player who violates the courtesy runner rule."  See also case CR1.
IOW, onetime is correct that it's a illegal substitute.
So, yes, stop it.  IF it's not stopped, then the player is out and restricted.  THe opposing coach doesn't need to wait for a pitch before asking about it (but if he asks "too early" then there's the risk that the umpire wil just change it in the name of game management -- so I'd at least wait for the ball to become live).