Why does an MP4 contain duplicate frames even though the camera says 60 fps?

Question

Frame-by-frame inspection shows repeated images in a '60 fps' MP4. Is that necessarily an editor bug, or can capture software create a nominal 60 fps stream without 60 unique frames every second?

RenderLab · Answer

The stream can be timed at 60 fps while capture delivers fewer unique frames. Screen recording under load is a common case: repeated frames maintain timestamps when new frames are not available.