Davddroid seems to have gone berserk, and decided to start duplicating calendar entries infinite times (well, it created several dozens to hundreds of them before I disabled sync in android). Duplicated entries have all the same data as originals (start and end time, description, recurring and alarms…)
The issue may have started while I was in the area with poor data connectivity (or it might not). Davdroid otherwise worked without problems for a year or so, although this same problem did occur once before (several DAVdroid versions back). Removing (either on phone or on owncloud) all the entires which were duplicated solved it that time.
All the problematic entries this time (3 of them) were created as recurring on android in default calendar app. Other entries (some of them also recurring) seem to be behaving normally (eg. not duplicating)
Davdroid is 0.8.3 running on Cyanogenmod9. Remote is owncloud 7.0.2.