Go far Tatkal. If you get confirmed tatkal accommodation then cancel it. If Tatkal is also wait listed, wait till any one of them advance to RAC or confirmed then cancel the other one. Make sure to cancel one of the ticket before chart preparation.
After chart preparation, you have to file a TDR and claim refund, which is slightly more complicated - note slightly - not herculean task.
Cancellation is a simple procedure. If you have booked ticket on line log on to the site, go...
more... to the booked ticket history, select the ticket to be cancelled, select the passenger to be cancelled. click on cancel. That is all. The amount will be credited back to your credit card account within 3 or 4 days.
If it is an PRS ticket, go to any PRS counter, fill up the form and get refund across the counter.
If you have an e ticket through an agent, call the agent tell him to cancel. He will refund the money to you.