As Acid said above, technically its possible to generate the same private key twice but the odds are so small its considered “impossible”. To add to that, you have the same odds using the QT wallet as you do a paper wallet generator.
The numbers crypto uses are huge, absolutely enormous. They are so large, in fact, that most people have difficulty understanding just how massive they are. One way I was explained was to picture a handful of sand. Look at each grain of sand. How many must there be in your hand? A lot right? Now picture a bucket of sand, picture a truck load, there are a LOT of grains, right? Now picture every grain of sand on EVERY beach in the world plus every grain of sand under all the oceans. If every grain of sand was its own Earth, with its own beaches and sand, then if you added up all these grains of sand on all these beaches on all these worlds… thats how many private keys there are. LOTS
