Prevent event conflict check jumping into infinite loop
Assuming that if current and next event occurence start time is the same then it is the same occurence.
It is better to ensure that get_next_occurence() returns None in such cases, but there must be a backstop from jumping into infinite loop too.
Test Plan: None
Subscribers: petersen, machniak, vanmeeuwen
Differential Revision: https://git.kolab.org/D785