Possibility to have 2 tickets on the same flight
 

I wonder if it is possible to have 2 tickets of the same person issued on the same flight? For example, if I already have redeemed a business class ticket and then later on award space for first class open up. Would I be able to issue another ticket in first class on the same flight?
If so, what happens when you check in? Do you get to choose which ticket you want to use?

But why would you do that? If you don't cancel the J ticket and use the F one instead, won't those miles (for the J ticket) go to waste, you then being a no-show in J?

I could save the J ticket for future flight by rebooking within validity of the the ticket. Worst case I could refund the ticket for $120/12000AM if I don't have the chance to use it.

From my experience is that there is no limitation of the number of tickets you can have for one flight. Especially if they were in different class, just tell the check in agent which one you're checking. I've done checking in the one that I did online check in with, and later changed the unwanted one.

I frequently book 2 seat with CX to have an empty seat next to me -- this no problem, the 2nd ticket is marked as "extra seat", and I only get miles for the seat I am sitting on :(

I think that's a different scenario. These bookings have a notation, as you said, that the extra seat is not occupied by a different person. OP is talking about having two different seats booked in two different cabins.

My answer to that is, as long as both are ticketed, CX can do nothing about it.

interesting that CX dont do anything about duplicate tickets?

QF will cancel one automatically... and are quite strict about it!

If the flight is full, wouldn't they be able to give the seat to someone else. They know there would only be one check in among the two tickets.

It is not selfish, airlines do oversell.

My experience on a couple of occasions is that there is something which alerts MPC/Asiamiles agents to conflicting bookings and they won't issue a second ticket on the same flight until the first one is cancelled. On one occasion I'd already ticketed my wife in PE on a flight when a J seat became available. I tried to go ahead and issue the J ticket (thinking I'd cancel the PE one a bit later) but MPC called back to say they'd noticed the "conflicting booking" and the PE reservation had to be cancelled before the J reservation could be ticketed.

I've been caught out on this too for AM bookings. But they don't stop duplicate revenue bookings?

I've always wondered about buying an empty seat next to mine. Also wonder if you have a higher chance of getting upgrades if Y is oversold (since you will be opening up 2 seats).