don't know about a day, but doubling in a month is very easy...i have been doing it myself...
example: for simplicity sake assume nifty futures 1 lot margin requirement is 50,000.
all you need to do is day-trade to earn 25 points a day....
25 points x 20 days = 500 points/month = 50,000 rupees/month = double money
it is very easy to make 25 points profit i feel...i usually make 60-100 points profit every day...