If you have a specific sum of money that you want to double, you can invest it in a plan which allows for a lump sum investment. However, you need to take care of the duration of the investment. For example, if the investment plan gives a rate of return of 8% p.a., you will need to stay invested for about 9 years.
If your risk appetite allows, and your only aim is to double your investment, you can look for more aggressive investments, such as equity stocks. Though, with such investments, you may not estimate the investment horizon as the returns are market-linked.