Does removing every 2nd duplicate frame worsens visual quality?

[nji9nji9]

I have some older movies (probably taken with cheaper devices)
where say every second frame is a duplicate.
(For expample technical it's 30 fps, but actually it's only 15 fps.)
When removing the duplicates and saving the movie,
it seems to play more "stuttering" than the orig version.
My question:
Does the decoder do some kind of "smoothing improvement" when playing
movies with duplicates?
If so, it would be better to leave the duplicates in the movie?

[poisondeathray]

I have some older movies (probably taken with cheaper devices)
where say every second frame is a duplicate.
(For expample technical it's 30 fps, but actually it's only 15 fps.)
When removing the duplicates and saving the movie,
it seems to play more "stuttering" than the orig version.

If what you say is true, then in terms of cadence there is no difference in the smoothness of motion.

But your analysis might be wrong, or perhaps it was improperly decimated, removing some unique frames

If you re-encode with a lossy codec, then there is quality loss - but not in terms of a cadence change assuming you used adequate bitrate.

Or maybe the saved format predisposes to playback issues. eg. AV1 or VVC are difficult to playback on older hardware and could stutter or drop frames during playback

My question:
Does the decoder do some kind of "smoothing improvement" when playing
movies with duplicates?

No

If so, it would be better to leave the duplicates in the movie?

What was your reason for removing them ?

[nji9nji9]

I did no formal analysis but just frame-wise looked at the movie (here and there).

The pattern was not exactly as AABBAABBAABB...
but with some disturbances like AABAAABBAABB...

I removed every second frame with VD2's frame rate menu.
(The disturbances shouldn't be any problem then).

When playing the saved result, it seemed (?) more stuttering.

I removed the duplicate to let reflect the fps the reality:
It is 15fps, not 30 fps.
Also for Deshaker filter to work correct.

[poisondeathray]

I did no formal analysis but just frame-wise looked at the movie (here and there).

The pattern was not exactly as AABBAABBAABB...
but with some disturbances like AABAAABBAABB...

I removed every second frame with VD2's frame rate menu.
(The disturbances shouldn't be any problem then).

When playing the saved result, it seemed (?) more stuttering.

There is no visual difference between 15 FPS unique frames, and 15 FPS unique frames duplicated to 30 FPS .

Because the pattern isn't exact, a more likely explanation is some unique frames were dropped. Verify the results of the decimation - Check frame by frame.

I removed the duplicate to let reflect the fps the reality:
It is 15fps, not 30 fps.
Also for Deshaker filter to work correct.

That's a valid reason - remove duplicates before application of any temporal filtering