Double rates for first time and regular rates on ongoing basis

I am doing Benefits configuration and my requirement is to have double rates for first time and regular rates on ongoing basis. Here we are using standard flat rate but when employee enrolls, first time we should deduct twice the rate and next month onwards it should be the actual rate. Any idea how this can be achieved, I haven't worked much on fast formulas. So if anybody has done similar requirement then please let me know how to do it. It would be of great help.