Can duplicate frames effect motion?
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.