Hi mohitgupta1994,
I don't think I've ever heard of anyone seeing a question from the OG13/GMAT2015 (they're the same book) on any of the GMAC Practice CATs, so you don't have anything to worry about there. IF you retake one of those practice CATs or use the software incorrectly, then you might see some repeat questions though. You'll also see those questions discussed in various online Forums, so you should be careful about how many random questions you use from these Forums for practice (because seeing the questions again when you take the CAT will make that CAT result unrealistic). Those GMAC resources are quite useful during one's studies, so there's a strong argument that you should get at least one of the books and the free software.
GMAT assassins aren't born, they're made,
Rich