Double filtering and single filtering have the same effect on the paper for a given grade, final prints should theoretically be the same either way.
Single filtering is simple but it requires you to adjust aperture or time with every filter change. Just like when you add a filter in front of the camera lens.
Double filtering allows you to keep the enlarger aperture and time settings more constant. The filter theoretically remains the same density. For small changes it can be quite handy, say to go from grade 2 to 2.5 dial in the filter and print, no new test strips.
I use single filtration because it has become really rare for me to need to adjust away from grade 2 when printing and because I find that with every contrast change I like a different exposure setting anyway and I use an enlarger meter to set exposure so its no streatch to reset aperture or time for whatever reason.