He even says explicitly "(never mind how we deal with uniqueness)" in the solution section and has already acknowledged that there are only around 2 million combinations to check.
ETA since he mentions the implementation is a translation of this Haskell implementation: http://blog.jle.im/entry/unique-sample-drawing-searches-with... presumably the uniqueness is done by updating the list of remaining possibilities for selection each time a new value is selected.
ETA since he mentions the implementation is a translation of this Haskell implementation: http://blog.jle.im/entry/unique-sample-drawing-searches-with... presumably the uniqueness is done by updating the list of remaining possibilities for selection each time a new value is selected.