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.
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.
Geiger rounds out World Cup season with flying finale
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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