This seems likely to be the same bug as https://bugs.kde.org/show_bug.cgi?id=403124, which has now been fixed for the KDE Applications 19.08 release. The fix is to remove any duplicate Akonadi resources (i.e. which use the same calendar file) at KAlarm startup. The user is also now prevented from manually creating duplicate resources via KAlarm's interface.
See the KDE bug report for details of the git commits which fix this issue. -- David Jarvie. KDE developer. KAlarm author -- http://www.astrojar.org.uk/kalarm
