The frame duplication is a feature not a bug. I am probably just going to leave as is, but going to 30FPS presumable would eliminate this effect entirely (unless in really low light it starts going to 15FPS). The only possible bug is the very high noise frame and having more than needed duplicated frame before the sensor switches to long exposure. But we should wait for official word, it might be expected.
If multiple frames are being duped when sensor is at 60FPS (and none are dropped) it would account for seeing a visual stutter.