Can duplicate frames effect motion? I understand that sometimes when editing, converting, and rendering new videos you can end up with duplicate frames. In other words, two successive frames can be exactly identical.

To what extent can duplicate frames affect the appearance of motion in a video? I've seen the charge that it can make motion appear faster or slower, but I don't really buy this as one frame in a 30 FPS video is only 0.033 seconds. One frame on a 15 FPS video might be noticeable, but again, I doubt it.
What effect you see will depend on the content, and your own perception. In a static scene dupes will have no visual effect, but they always lengthen the clip.

With motion in the scene, a duplicated or skipped frame might be quite noticeable. I usually find them visually disruptive, but others might not -- it depends partly on what you're trained to see, and how engaged you are in the content.

On the other hand, a repetitive sequence of duplicated images will generally not be disturbing, though a trained eye may see it. This is the essence of 3::2 pulldown, for example. That doesn't serve to lengthen or shorten the scene, but to convert a taking frame rate to a presentation frame rate.