I turned up at the lounge in FRA once and the lounge agent commented (perhaps indiscreetly but it’s not the point here) that there was someone with the same name on my flight.

Brief analysis revealed that this imposter had the same frequent flyer card number as me too. Turned out there was no imposter and I had indeed booked myself twice on the same flight.

Not sure if that counts as fraud or not but clearly the airline didn’t bother to act on it, if they even detected it.

An add, which might make a difference: one booking was made directly with the airline and the other was via an agent.