Please describe the problem:
I have a recurring appointment (Every 1 month(s) on the third Tuesday forever) in my calendar which is marked as busy. When I uncheck "Show time as busy" and save the appointment (All Instances selected) I get the error message "Object ID already exists". It's not possible to save this change and I have to discard.

Steps to reproduce:
see problem description

Actual results:
error message "Object ID already exists" and nothing more

Expected results:
no error message and saved changes

Does this happen every time?
yes

Other information:
I tried two different environments. The error happens on both:

holger@degenatron:~$ env|grep ^LC_
LC_PAPER=de_DE.UTF-8
LC_ADDRESS=de_DE.UTF-8
LC_MONETARY=de_DE.UTF-8
LC_NUMERIC=de_DE.UTF-8
LC_TELEPHONE=de_DE.UTF-8
LC_COLLATE=de_DE.UTF-8
LC_IDENTIFICATION=de_DE.UTF-8
LC_MEASUREMENT=de_DE.UTF-8
LC_CTYPE=de_DE.UTF-8
LC_TIME=de_DE.UTF-8
LC_NAME=de_DE.UTF-8

holger@degenatron:~$ env|grep ^LC_
LC_ALL=C