Similar thing happened to me on a QF flight from MEL to BNE last year....machine went beep at the gate and I was told that I had already boarded! Agent looked very confused (Seems to be the standard response!) but sent me down the airbridge anyway. My seat was unoccupied and I thought no more about it. No idea what actually occurred, but the security implications are pretty serious, although this never occurred to me at the time.I also have a pretty unusual surname, so I very much doubt it was a passenger with the same name.