Service tax on gta

My client(A)  is hiring GTA (B) and providing services to its customers(C). But they(A) are claiming it as transport charges say Rs 10,000 paid in profit and loss A/c. Also they are recovering the same including their commission of Rs 15000 from its customers(C) and showing it under Sales in profit and Loss A/c and earning commission of Rs 5000/- in the transaction. C is paying Service tax under  RCM on Rs 15000/-.Whether in this case B is also liable to pay service tax on RCM basis on Rs 10,000/-??? Will this not lead to double taxation on part of B if C is paying service tax on entire Rs 15000/-????