CAN - duplicate message at receiver possibilities?
The presentation you linked to talks about a situation where a receiver accepts a message, but a transmitter recognizes the same message as incorrectly transmitted. Currently I don't understand exactly how that would happen, but I guess a receiver could use data from a frame where the ACK bit does not get set for any reason, which causes the transmitter to resend the frame.

IMO the exact reason why this can or would happen is irrelevant, but instead you have to think about the kind of data that you are transmitting: if you periodically send e.g. the current value of a sensor reading over the bus a retransmission will not be a bad thing. If on the other hand your message contains a small chunk of a larger data block then resending that part will cause the larger block to be corrupt. This is the reason why protocols like CCP or XCP contain a message counter.