How filters on whole track + filters on clips are added

GoPro footage can vary a lot with light / dark clips. I might put brightness and contrast filters on the whole track on the timeline. If I then add a brightness filter to an individual clip how does shotcut handle the filters for that clip? e.g. brightness 110% on whole track plus an extra brightness of 110% on a clip. Would the result be effectively 120% ?

Yes, redundant filters combine effort. You can add most filters to the same object multiple times as well.