Someone obtained my boarding pass and got on my flight
 

This is a first for me. I arrived at Gate 31 at Schiphol this morning at 09:50am for the BA429 to LHR. I arrived earlier this morning on the Cathay flight from HKG, bags checked through to LHR. My boarding pass was also issued in HKG and I have seat 4A in Club. When my boarding pass was scanned at the gate it beeped and the message ALREADY BOARDED lighted up on the scanner. The gate agent looked confused and didn't consult her colleague but allowed me on. I entered the cabin to see someone already sat in my seat. I advised one of the cabin crew that there was someone in my seat, who then promptly asked to see the guy's boarding pass.

The cabin Manager was then informed that there were two boarding passes, both in my name, for this flight. I showed my passport to confirm my identity and the guy was also asked for his passport, in which a KLM boarding pass was found, tucked inside, for a flight to LHR which had already departed. He had originated in Uganda and was connecting to LHR, all on KLM. How on earth did someone obtain my boarding pass and pass gate security boarding check at Schiphol to get onboard this flight? The Captain was informed and he was offloaded. The cabin Manager said she would write a report on this and actually handed me the duplicate boarding pass the guy had obtained. My name is quite unique being of Vietnamese origin. What should I do as I am most concerned that someone managed to board an aircraft using my boarding pass. I've checked my Executive Club account, my Avios are still there and it doesn't look like my account has been compromised. My booking for this flight was an Avios redemption.

Edit.

I hope offloaded in to the arms of the police. We do hear of errors occasionally when someone on the same flight is incorrectly issued with the wrong bp at check in, but from the story it seems this person was never supposed to be on your flight and therefore there are questions about his intentions and motive which need answering.

I agree, this is deeply concerning. If the police didn’t collect this impersonator, then the OP should surely report this to the police?

edit.

I don’t think Schiphol security would be involved here: presumably an unclean origin so would have cleared security again in AMS, but no boarding pass scanner IIRC. This is on BA at the gate certainly for not matching name to passport.

Im assuming OP didn’t drop his boarding pass and acquire a new one…?

I agree that on the face of it, it is nefarious and potentially quite serious. But at a big airport like AMS I can imagine the Dutch Coastguard may not be that bothered, indeed would rather the individual concerned would leave Schengen somehow. The most innocent reason for this to happen would be if the OP had checked in, in AMS, and there was a paper jam for the boarding pass. The agent is well practiced to extract the card. What sometimes happens is that the next time the machine is used it reprints a second copy of the jammed card, and this gets handed to the next but unrelated customer. The next most innocent reason is that the person here missed their flight on KLM, KLM rebooked on BA, passenger went to BA desk, BA desk pressed the wrong buttons. But I have to say I feel I'm clutching at straws here - something more sinister may well be at work here and it's good the OP, and BA, ensured they were not on this flight.

This is a puzzle in so many ways, it's almost as it was indented to check security, i.e. BA did this themselves.
The barcode on a boarding pass s quite unique, so it have had to been printed at either BA service desk, or any other airline that have the rights to issue boarding passes on behalf of BA (Some/All One World partners?)
I very much believe that this man from Uganda had to use his KLM boarding pass otherwise he would not have been able to transit at Schiphol and pass security.
He just missed his flight and took a chance, and the Gate Agent for some reason let this slip.

Unbelievable on every level.

Not nearly the same but I recently tried to obtain my BP for a LATAM flight in their app, in the process I noticed it “glitched” (froze for a few seconds vs the usual processing screen) and then showed me the BP for a completely different person on a different flight from a different airport (our booking references were also completely different). Obviously didn’t save it to my phone but it’s a pretty bad data / safety breach!

I'm wondering why the decision was not taken to remove everyone from the aircraft, and their baggage and to rescreen? Were people asked to reconcile the baggage overhead?

I agree with others that it is a major and inexcusable security breach. I would also be tempted to report to the police, but I would also send a formal complaint to BA. I would do that using real mail and recorded delivery and ask for a full investigation and explanation as to how this happened mentioning worries relating to identity theft.

Whilst it is always possible that this stemmed from some improbable succession of random bad luck, this is to serious to assume the best.

This is highly irregular – at least two failures here in the form of the passenger coming upon a boarding pass for another passenger, and the passenger's passport not having been checked properly against their boarding pass at the gate. The latter can happen with gate agents on autopilot, I guess. And cws mentions a way for the former.

I don't think "security" will come into question as the passenger will have been screened going airside like everyone else. As he was a connecting passenger from non-Schengen he would not have been checked by the Koninklijke Marechausee, although I suspect he will have made their acquaintance shortly afterwards. I doubt anything will have come of this besides the passenger being helped on his way out of Schengen by the said KMar and made someone else's problem.

There is no material chance that the OP will get any feedback on what action if any has been taken, due to GDPR with respect to the action taken against the passenger or the staff, and due more generally to it being security sensitive information.

What I fail to comprehend is that passports with Boarding passes have to be shown in order to board a flight to London. Surely this should have been picked up at the gate? If name and photo the passport and Boarding Pass do not match surely they not have got past?

It would appear that this is a very serious matter indeed.