ffmpeg DTS output issue creates double length files

I am attempting to concatenate mp4 files with the following command line.

ffmpeg -fflags +genpts -y -f concat -copytb 1 -safe 0 -i .list.txt -c:a copy -c copy $outputFileName

I get a number of errors on these files.

Auto-inserting h264_mp4toaexb bitstream filter
Auto-inserting h264_mp4toaexb bitstream filterx

And more imporantly a lot of these errors.

[mp4 @ 0000000002574720] Non-monotonous DTS in output stream 0:0; previous: 1628167, current: 1603584; changing to 1628168. This may result
in incorrect timestamps in the output file.
[mp4 @ 0000000002574720] Non-monotonous DTS in output stream 0:0; previous: 1628168, current: 1604096; changing to 1628169. This may result
in incorrect timestamps in the output file.
[mp4 @ 0000000002574720] Non-monotonous DTS in output stream 0:0; previous: 1628169, current: 1604608; changing to 1628170. This may result
in incorrect timestamps in the output file.
[mp4 @ 0000000002574720] Non-monotonous DTS in output stream 0:0; previous: 1628170, current: 1605120; changing to 1628171. This may result
in incorrect timestamps in the output file.

The individual files seem to play just fine, and the final output file plays, but has a length exactly double that of the input files.

In other words when I get to the "end" of the video, the player reports that the video still has half way to go.

i am happy to re-encode the audio if necessary to correct this problem. any suggestions on how to correct it are welcome.