Any time I see words like "at least", or "any pair", I immediately think about reversing the question!
In this case, it is esier to find the probability of ALL the rolls being different, and then subtract this from 1. You avoid the danger of missing counting one of the ways of accomplishing...
The simpest form of chain hoist consists of a pair of discs of different diameters, locked together with their centres aligned. The two gears on the pedal crank of a ten-speed bike are what you should picture. The gear "teeth" are designed to mate without any chance of slipping with the chain...