This is a quote I use very often! As with many other great quotes, I think it originates in English from Trevanian's Shibumi (when Otake-san, the Go master is giving his final teaching to Nicholai):
Do not fall into the error of the artisan who boasts of twenty years experience in his craft while in fact he has had only one year of experience — twenty times.
Most probably Trevanian has drawn upon some other source, tough I've never been able to track that.
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