Prevent event conflict check jumping into infinite loop

Summary:
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

Reviewers: vanmeeuwen

Subscribers: petersen, machniak, vanmeeuwen

Differential Revision: https://git.kolab.org/D785