PPF instalments per month?

I have a PPF account in Bank of India and I have deposited two instalments(Rs. 50,000/- each time) in one month only, that is April,2012. I know maximum subscriptions should not exceed 12 per year. But are two subscriptions allowed in one month, as I have done. If I have made a mistake, how to rectify it now? The bank has raised no objection.