Looking at those files makes me think of a possible installation error: that one or more partitions on the old install were used and mounted "without reformatting" for the new install. Is there a timestamp check that could be performed (install time/date for the file, rather than the datetime that ls -l shows?
Could be. After I removed Windows I had several attempts to boot into Debian as the GPT gave me some headache. It is possible that I made a mistake and did not format the partition every time.
How could I do this timestap check you mentioned?