ffmpeg flash frames last still image in concat sequence
Based on the discussion at the bug tracker, this should be a workaround:

file path/to/001.png
duration 5
file path/to/002.png
duration 5.96
file path/to/002.png


and 

ffmpeg.exe -f concat -safe 0 -i path/to/input.txt -vsync vfr output.mp4


The idea is to duplicate the entry of the last image. If you need the duration to be precise, then decrease the duration of the now 2nd-last entry by 1/fps seconds. For 25 fps video, that's 0.04s. If you don't do this, the output duration will be 0.04s longer.