Normally FR calls / emails the passenger if there are two bookings for the same name on the same flights. My friend has the same name as his dad and they have gotten a call when they are travelling together to check it is indeed two separate passengers.
Mind you, if there is any internet problem it is normally advisable to call them immediately after the error has been made - they tend to be more "helpful" (ie more likely to cancel and refund one booking made in error) if contacted immediately.
XSB