How do i make a double sided polygon?

[Airwolf]

i have come accross the problem of not being able to see the floor or walls of my chopper from the inside, how do i fix this? i assume you would make the faces double sided but how?

[Erex]

select face then copy it and press W
but you shouldnt have 2sided faces cause it can be problematic is better to have some space between this faces