Am I correct in assuming that when you set the shutter to less than 1/24 it just inserts duplicate frames? That is 1/12 recorder at 24p would just capture 12 frames then play each one twice. at 1/8 three identical frames in a row etc.

I've not done it with 24f, but at 60i it does exactly as you suspect. At 1/15, you'll get two identical frames. I've counted them on my NLE. At 1/8 you get about four frames. I haven't counted those, but extrapolating the math that's what you should get.