Hi
As far as I know you can't. I don't know why this is, but its probably a complication that arises when your clip is loaded and rendered for editing. Possibly the clip loaded is rendered 40 fps. On the other hand it may just be that using the Next frame option moves to the next nearest frame to 0.025 sec. hence the occasional duplicated image NCH may clarify this.
As it stands, advancing a frame at a time will move along your timeline by 0.025 seconds irrespective of the frame rate set by your camera. This means on the timeline 40 frames are viewed/sec. If the actual clip was shot at 30fps then every second VP must duplicate 10 frames. So on paper, every 1/10 of a second (4 frames) a duplicate is created to keep the VP framerate correct with the timeline markings. As this is not totally accurate every so often a duplicated frame is not made as the timings are correct at that instant ..etc.
However when the edited video is exported the original framerate is used or is adjusted to the value you set in the output values window.
When it comes down to it, I don't think you will see any duplicated frames at 1.40 sec..or even 1/30 sec as ones eyes have a persistence of vision of about 1/16 sec (0.062 sec) so you could easily edit to 2 frames (0.05 sec) and see no difference. The 0.025 minimum available by VP is well within this tolerance
Nat