The permanent urls for 2015-01-25-stick-wochenende.html and 2015-02-22-stick-wochenende.md are both https://raumzeitlabor.de/blog/stick-wochenende/.
At the moment this is the only case as can be verified by
ls app/_posts | sed 's/^[0-9]*-[0-9]*-[0-9]*-//g' | sed 's/\..*//g' | sort | uniq -c | sort -h
Renaming one of thes posts would solve the issue but we have recurring events like GPN, BMI, Mettwoch, LHC, which are likely to raise the issue again.