Why some events are duplicated on my Notion?
When you edit a recurring event and apply the changes to "This and all the following events", it will consider the new occurrences as new, will "remove" the old ones
This is the standard behavior on Google Calendar, as the updated recurring events are considered new.
One option to prevent an unwanted deletion, is to turn off the setting "Allow event deletion" on the Sync Settings, and delete them manually if you do not want to lose the custom data you provided on Notion.
We are working on finding a stable workaround to link recurring event changes between them.