Sarvice tax calculation to avoid double taxation

Party A provides service.
Service includes service provided by Part B.
Party B provides invoices including service charge.
Total invoice amount is Rs. X

Party adds its service charges of Rs Y
Total invoice to customer of Part B should be:

a. (X+Y) + Service Tax(X+Y)? [Note: Amount X already includes Service Tax]

b. (X+Y) + Service Tax(Y)?

c, Any other [Please explain]