The 24-year-old Norwegian pulled out all the stops to maintain the gold standard after trailing Germany's Juliane Seyfarth after the first run.
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Seyfarth gathered 122.2 points from her first effort but could only muster a score of 115.6 in round two to leave her in the bronze medal position and handing the initiative to Lundby to strike again.
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The 2017/18 overall World Cup winner saved her best till last to soar 96.5m and glide to an unstoppable 127.6 points, claiming the title with a total score of 247.6.
Germany's Carina Vogt secured second spot on the podium 7.3 points behind Lundby as Austria's Eva Pinkelnig finished fourth.
Back-to-back victories in Rasnov secured the Norwegian top spot in the overall season rankings as previous leader Katharina Althaus could only manage a fifth-place finish in Sunday's event.
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