dear friend
in case the person (contractor) is one with whom u have done business(contract) in previous years and there the payment exceeded the amount of 75000 or 30000 as the case may be , in such a case u will deduct from the very beginning since u knw that payment will go beyong the limit
rest ii case ur last payment exceeds limit of 30,000 but without exceeding 75000 then tds only on the last bill
but where my friend ur last payment make the net payments total beyod 75000 then in such case tds will be deducted though only on the last bill but will include the amounts of all the prvs bills as well
hope ur query is cleared :)