Hide Clone Events
If you sync your calendars with OneCal, every synced event exists twice: the original, and the clone that Calendar Sync created on the other calendar. In the Unified Calendar View, where all your calendars are shown together, this would make every synced event appear twice.
The Hide Clone Events toggle at the top of the calendar view sidebar solves this: turn it on, and clones created by OneCal are hidden from the view, so you only see each event once.
Good To Know
Hiding clones only affects the Unified Calendar View. The clone events still exist on your calendars at Google, Outlook or iCloud and keep blocking your availability as usual. When printing your calendars, you can choose separately whether clones are included.