Note that the last week’s weekly expiry contract is nothing but the monthly expiry contract for the indices options. There are no different weekly and monthly expiry contracts for the last week of the month.
As the last weekly expiry is considered as the monthly expiry itself, NIFTY29DEC18000CE and NIFTY22DEC1800CE will have the same premium. You can verify the same in the transaction details of the backtest results.