most cars have internal filter.
external filters are added for more runtime.
it's quite common to run dual filters with the actual filtering element removed just to get more runtime as you can get more fuel in the car.
however official rules clearly state that total volume of FUEL system can be 125cc for buggy and 150cc or truggy. no idea if they actually measure it on EFRA/ROAR/IFMAR races as i seen lots of photos with cars having multiple filters