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That's by design... it was designed by a mathematician named Arto Inkala.
Supposedly, it takes one to be able to think 7-8 moves ahead to be able to solve this one.
I hope you did not spend too much time on this. Trial and error is the common strategy to solve this one by expert sudoku solvers.
I still spent half an hour but only managed to put in these two numbers for sure. Other numbers would be a trial and error, as you say.
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