A quarter of a century ago, KTM took the public by surprise by launching the KTM 620 DUKE. At the time, the Austrian manufacturer was known for its off-road motorcycles, so no one was expecting the release of a radical street bike. Today, all KTM Naked Bikes bear the name DUKE, from the KTM 125 DUKE right through to the KTM 1290 SUPER DUKE R…