Google-calendar – Duplicates in Google calendar

There's an article on this here. I've not checked that it works, but it seems pretty straight forward. The essence is that instead of importing the calendar, you add the URL of your event feed as a public calendar - here's a summary of the steps:

1. In Facebook click on "Events"
2. Click on "Export Events" and copy the URL
3. In Google Calendar go to "Add" under "Other calendars" and choose "Add by URL"
4. Paste the URL you copied from Facebook and click "Add"

[Update: to make answer more complete]
If you run into issues with private events not showing properly then check out eventbusyfix.info. This site acts as an inbetween to clear the private flag and enable Gmail to show them properly (Thanks to prestomation for this tip)
[/Update]

Hope that helps!

You can do this by clicking "Add | Add by URL" at the bottom of the left nav on your google calendar homepage.

Figure: Adding an iCal to sync is as easy as adding the URL

You will be asked to choose to sync with your main calendar or a new named one, but if you stick it in the main calendar you should then get it on your other devices.