I may be wrong, but every time you do a check-in you are giving your information and they record it, and you have to show the same document at boarding time.
This doesn't mean that it must be the same document that you used to enter or exit the country though, although depending on the destination the airline may require proof that you can enter the destination country, like a visa or a passport, because having a passenger refused entry may be an hassle for them.
You can use a document to exit the departure country, another for the airline (with the caveat above) and another one for the destination, even with different names on them.
You would should both to the airline staff. "This is my ID I used, and this is my ID which allows me entry so you(the airline) won't have to bring me all the way back to my arrival point." Airline staff will not record this second form of ID: it's only to show the staff for a moment.
Okay, but spies, terrorists and tax evaders (or military duty evaders for Iran) with double nationality have been using the double-passport trick for over a century: Is it time that airlines feed their incoming passenger list to the destination authority?
I mean… isn’t that built-in to the system already? I never supposed entering Thailand that I wasn’t already known to the Thai border police, who surely must have checked that I’m not a banned and/or wanted criminal, right? Or is this system supposed to a single safety net of the destination’s border agent recognizing a fake passport just by checking the numbers in a DB? Is that why TPB’s founders escaped to Thailand, is this why Wolkswagen’s pollution manager thought he could cross the USA freely on holidays without spending 8 years in jail, is this why Carlos Ghosn escaped Japan?
Or do countries carefully avoid reaching an agreement on airline IT systems, just because they do need each others’ spies to cross freely?
It's not even a "trick". It's just normal for dual citizens to make things faster.
Leave Canada on Canadian passport. You just get nods and wave throughs. Enter European country on EU passport in the smaller line (or nowadays automated border control stations). Just scan passport, (don't) smile for the photo and off you go. Flying back Canadian passport is on file and the CBSA just asks you some basic questions and waves you through.
As far as I know if you're a citizen of a country, you have to use your passport from that country to enter/leave that country, at least it's that way with USA:
[1] > U.S. nationals, including U.S. dual nationals, must use a U.S. passport to enter and leave the United States.
And the German foreign ministry says it's international practice to do so, but says "should" instead of "must":
[2] > Bei Doppelstaatern (Erwachsene und Kinder) ist zu beachten, dass nach internationaler Übung
> - die Einreise nach und die Ausreise aus Deutschland nur mit deutschem Reisepass oder Passersatz, z.B. Personalausweis
> - die Einreise in und die Ausreise aus dem anderen Staat (dessen Staatsangehörigkeit die Betreffenden ebenfalls besitzen) nur mit den nationalen Dokumenten des anderen Staates
> erfolgen sollte.
Also a requirement for Canadian dual citizens [3]
I guess it'd be interesting if one has a EU passport but is entering the EU through another Schengen country (eg. A dual Canadian - Italian citizen flying from Canada to Germany). Logic says keep it simple and show the EU passport, but is there a EU clause somewhere that requires that?
Like I said, and this is personal experience: yes Canada needs your Canadian passport on file so they (and thus the airline) will let you board/back in.
I have been to several EU countries and none of them cared for the Canadian passport. Tried showing both at the beginning. They don't care. So we only show the EU one now.
Also funny: showing "the other passport only" because you don't have a current one. Like try using the Canadian one if you don't have a current EU passport (expired, never had one yet etc). Takes longer. Works too. Haven't tried recently so things might have changed.
Leaving an EU country has never required the EU passport. In fact having the Canadian on file is a requirement. Otherwise they call your name at the gate because you neither have a Canadian passport nor an electronic authorization on file and thus they can't let you board. So doesn't work in reverse nowadays.
For all the airline actually cares you should be able to just board a plane as long as you paid for the ticket ... everything else is what they are made to do or do because of (dis) incentives.
This doesn't mean that it must be the same document that you used to enter or exit the country though, although depending on the destination the airline may require proof that you can enter the destination country, like a visa or a passport, because having a passenger refused entry may be an hassle for them.
You can use a document to exit the departure country, another for the airline (with the caveat above) and another one for the destination, even with different names on them.