Travelling with dual passport and changing at airport entry

As I have noted, there is an answer that explains the general principle

Your “complication” is that, even if you booked the flights on one ticket, you cannot check in for the flight to the UK on your Indian passport, as you have no Visa. You can check in for Doha, as you could enter there on your Indian Passport.

Whether or not you need to leave the transit area will most likely depend on wether there is a check-in counter for your connecting flight in the transit area (which may depend on the flight and airline, I’m not familiar with Doha specifically).

In case you do need to go through immigration, better use the Portugese passport. The Indian should work as well, but you’ll end up with multiple sets of stamps, which might raise an eyebrow on your return to India.

It is very unlikely that immigration will check if you left on the same passport, and if they notice it us unlikely that they care, there is no law that I know of that prevents you from showing any valid Passport (in Doha) that you choose.

In any case, you must check in for the next flight with your Portugese passport. The airline won’t care what stamps it has as long as it is valid for the UK.

When you arrive in the UK, you show the Portugese passport.

On the way back, you repeat the procedure but now you should go through immigration in Doha, even if you don’t have to. Use your Indian passport. This way you have it stamped when the Indian immigration checks it.

If we assume that you will have to pass through immigration for some reason (which you may not actually have to do) and that Qatari immigration officers want departing foreigners to show the same passport they used when they entered (which is probably the case), the question boils down to this:

Do Qatari immigration officers check passports against the manifests of arriving or departing flights?

If the answer is no, then you can use either passport. If the answer is yes for arriving flights, no for departing flights then you should use the Indian passport on your way from India to the UK. If the answer is yes for both then you may have a problem.

However, I have flown several times from the US to Bosnia and Herzegovina by way of the European Union. I have always shown my EU passport in the EU and my US passport in Bosnia, and it has never been a problem in either place that the other passport was the one on the manifest. This doesn't say much about Qatar, obviously, but it does show that such checking is not universal.

Furthermore, Qatar should not much care about your two passports nor about your visa status at your destination (that is normally only the airline's concern). If you show the Indian passport at the exit check on your way to the UK, they probably won't even notice that you don't have a visa for the UK. If they do, you can explain that you also have a Portuguese passport, which you will use there. Unless Qatar has some diplomatic agreement with India where it has undertaken to detect Indian citizens traveling with other countries' passports, that will be the end of the matter.

One other word of advice: I once checked in for a flight from Nairobi to the EU with my EU passport. At the gate, we were asked to show our passports with our boarding passes before getting on the plane. The security agent took my passport and became very concerned. I realized I had handed him my US passport, so I said "I also have this other passport, if that makes any difference," and showed him my EU passport. He immediately relaxed and pointed to the security sticker on the back of the passport. His role, it seems, was to double-check not only the identity of the people boarding the plane, but that they had been given the appropriate sticker at the security screening.

(This trip started in Rwanda, by the way, where I had used my US passport. Rwanda also did not care that I had shown my US passport at the exit check after checking in for the departing flight using my EU passport.)

If you get such a sticker on the back of one passport, you will probably have to show that sticker to someone else at some point. Trying to peel it off and stick it to the other passport is probably not a good idea.