Given yours and clarence5ybr's rely I think the computer might only look at seat assignments and unless there's 2 or more pax assigned to the same seat, it won't flag anything. Since she had the same seat for the 2nd and 3rd legs (flight numbers were different), the computer didn't notice anything out of the ordinary. When we got to MEM, I asked an agent who said she was shown on the system as having flown the DTW-MEM leg.

If the system did look at name, I'd imagine any pax who shouldn't be onboard would be caught at boarding.