Selecting one every n frames from a video using ffmpeg
The image2 muxer defaults to constant frame rate. So if the input is, say, 30 fps, and you select every 10th frame i.e. frames with timestamp 0s, 0.33s, 0.66s.. then ffmpeg will duplicate frames to match the input rate so duplicate 9 frames for every input frame.
Way to avoid that is to set video sync method to passthrough or variable frame rate
e.g.
ffmpeg -i input.mp4 -vf "select=not(mod(n\,10))" -vsync vfr 1_every_10/img_%03d.jpg
This can potentially affect the full extraction if the input is VFR. So, use
ffmpeg -i input.mp4 -vf select -vsync vfr all/img_%03d.jpg