Calendar - Non repeating events displaying in multiple months, how to fix it?
It would be a guess as its hard to say without more details but its most likely to do with contextual filters not set up correctly. For month I have them set up as follows:

Provide default value: current date
Granularity: month
Dates to compare: Start/End date range
Add multiple value identifier: Yes
Content: Dates - start date (field_event_date)
Method: OR

For week its exactly the same except using week granularity and so on for days and years. Make sure you dont have any other calendar views (even disabled ones were playing tricks on me before). Also make sure your sort is based on the same date field as the contextual filter.