[FFmpeg-user] Duplicated frames warnings when processing video after pipe concatenation but not when processing source files

[Joel Roth]

Hi list,

First of all, thanks to the ffmpeg developers and community
for this awesome software.

I'm using a two-pass workflow, where I pipe concatenate
source files in the first pass, then trim and process the
result in a second pass.  I am not changing framerates. 

Quoting from https://video.stackexchange.com/questions/20958/ffmpeg-dropping-duplicate-frames

    FFmpeg duplicates or drops frames when the input and output framerates differ. 

If I apply an ffmpeg command line to process a raw MTS file
from a camera, I do not get "More than 1000 frames duplicated"
warnings during processing.

However if I combine MTS files using pipe concatenation,
e.g.

    ffmpeg -i concat:"./00000.MTS|./00001.MTS" -codec copy ./00000.mp4

then run the same command line on the mp4 result, I get
plentiful duplicate frame warnings.

I'm using Jon Van Sickle's statically ffmpeg, to wit, ffmpeg-git-20180216-64bit-static/ffmpeg
from https://www.johnvansickle.com/ffmpeg/

The command line is:

    ffmpeg -i ./00000.mp4 -to 3:31 -c:v libx264 -preset:v medium -profile:v main -tune:v film -s:v 640x360 -c:a aac -b:a 80k -ac 2 -ar 24000 -level 3 -strict -2 -copyts  -ss 1:29 ../final/20180220-1.mp4

The rendered video appears okay, but I would like to
understand what is occurring before running a larger
rendering job.

Thanks for any suggestions. 

-- 
Joel Roth