Hi Suzanne,
The Merge Duplicates button will merge duplicate Venues and Organizers, but not duplicate events. If you are seeing duplicate events from Facebook, that may be a Event Aggregator-related topic, and we can troubleshoot that issue.
If that is the case, can you provide me with the URL that you are trying to import, so that I can do some testing on my end?
Also, can you enable and check the Event Log under Events < Help to see if you see anything unusual? Right under that is your Event Aggregator Status. Can you take a screenshot of what you see there?
Let me know if you have any other questions in the meantime!
Thanks,
Jaime