Please, open a bug in the report category describing your problem : version, number of calendars… You can also follow what I wrote there: VALARM-loop of jolla-alarm-ui to list and edit your alarms. It may help to understand the issue.
To clarifiy: Multiple reminders are gone since I updated to 184.108.40.206. At least, for me, as of now.
Above I posted Improved Calendar Design a workaraound that worked for me in the meantime after sync stopped working. Using the solution linked above, sync worked again, but (up to) tripled the reminders.
Now, in 220.127.116.11, everything is fine. ONE reminder per date.