United (and many other airlines) do this frequently for a number of reasons, including situations like yours.
The simple answer is that yes, you have a "confirmed" reservation on both flights - but you only have a ticket on one.
To understand exactly what they are doing you need to know the difference between a "reservation" and a "ticket". A reservation is basically your booking record. It's created when you first select your flights, but before you pay for them. It will include not only the flights that you're currently booked on, but also things like waitlisted flights, etc. Once a flight is added to your reservation in a "confirmed" status then basically a spot is reserved for you on that flight.
However before you actually use that reservation you need to pay for it. When you do that, a "ticket" is issued, which includes details of the flights you are confirmed on. Historically this was a physical piece of paper, but now days it's (almost exclusively) an electronic e-ticket.
If, for example, you make a change to the reservation (eg, you change to a different flight, or a waitlisted flight become available) then this change is first done on the reservation, and then your ticket is "re-issued" or "exchanged" which historically was the act of you giving back your old ticket and being given a new one with the new details on it, but today is just a change in a record in a computer.
Now, back to your question. What United will have done for you is to add a new flight to your reservation, which would have almost certainly have been in a "confirmed" status, but without removing the flight you're currently booked (and ticketed) on. So at this stage you're "confirmed" on both flights, but you only have a ticket for one of them.
If you manage to make it to your original flight (as you did) then the extra segment on your confirmation will be canceled (sometimes by an agent, sometimes automatically, or sometimes simply because a ticket was never issued for it by the time the flight was ready to leave), and it's as if it never existed.
However if you miss your original flight then the United agent will remove the segment you missed from the confirmation, and then re-issue the ticket without that segment, but now with the new Qantas segment. At this point you have a valid ticket for that new flight, so you're ready to fly!
If you had attempted to view the reservation on the United website after the additional segment was added you would have seen both flights listed, along with a message that your reservation has been modified and that you should contacted United to have your ticket re-issued, which is an indication that your ticket didn't match your reservation (in this case, as expected!)
Depend on who you're talking to this process can be referred to as "double-booked" (as you have two bookings between the same locations on different flights), a "backup booking", being "protected" on the later flight, or probably any of a dozen other terms. It's a process I've been through a lot, most recently last night due to a combination of delayed and overbooked flights!