Duplicate frame where there shouldn't be duplicates?

I processed a video of feature film duration with constant 25fps using Chronos Fast for frame interpolation and Proteus manual for enhancement to 1440x1080p50. I converted the exported file with FFmpeg targetting constant 50 fps and found that ffmpeg creates 57 duplicate frames at apparently random positions. Assuming that frame interpolation with VAI targets strictly constant frame rates, the issue indicates that there’s something is wrong with some timestamps in the stream created by Topaz.

Has anybody seen such a problem?
Does VAI create any kind of verbose log while processing, which could help to reveal the cause of the problem?

Edit: I also converted the original video with ffmpeg to verify that it has strictly 25 pfs and it has.

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Update.

To narrow down possible causes, I exported the same source video, without enhancement and up-scaling, just with frame interpolation Chronos Fast 25->50 fps twice: The first export with ‘Duplicate Frames Replace’ enabled (Sensitivity 10), the second with ‘Duplicate Frames Replace’ disabled. Then I converted both with ffmpeg for constant frame rate.

With ‘Duplicate Frames Replace’ disabled, ffmpeg doesn’t create unexpected duplicate frames, enabled however, it creates the same number of duplicates as in my production run.

Consequently, this issue is caused by the duplicate frame detection and replacement.

I tried to reproduce it with some clips of short duration containing some duplicate frames, but failed. Unfortunately, I cannot share the source video, but metadata and conversion logs, if that’s o any help.

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