If the casino did the cross promotion to another casino in the same group you should get credit for it.. If you hadn’t send the player to the first casino, no way that he would have recieved the marketing material..
Only thing I could think of that might create this situation is that you player visited “the other casino” before through another affiliate. But than it’s going to be a difficult disscussion, so I’m staying with my first thought.. 
Oh and I do know of programs that have different rules as to revshare or cpa players.. There, cpa is only given once, no matter how many casinos in the same group the player signs up. But rev-share is given when crosspromotion is done.