One of the quirks of the X100 camera is that if you want to shoot the camera in RAW (only) mode you cannot turn off the red eye reduction pre-flash. This means that even with the camera's built-in flash or the external EF-20 unit - there will be a double flash when exposure is made. I am not a fan of this since it drastically slows down my exposure time (and confuses people who move after the first flash).
Until the new firmware upgrade is released later this month the only way around this is to set the camera to record a RAW + JPEG file - and then you can turn off the red eye reduction pre-flash!
I know we are all excited to hear the details of the new firmware and we can expect that in two weeks (approximately).