That itinerary is not a problem. It would only be a potential problem if the nested trip was B to A to B and was designed to avoid a minimum stay requirement, such as a Saturday night stay. Even then, with so many fares now being one way, it is less of an issue than it used to be. And finally, if the B-C-B trip is on a different airline than the A-B-A trip, there would absolutely be no issue.