TUESDAY night’s showdown between Adam Lambert and Kris Allen was the lowest-rated final singoff show in “American Idol’s” eight-season history.

The show averaged 23 million viewers from 8 to 9 p.m., down 15 percent from last year’s final singoff between David Cook and David Archuleta.

It was also down 18 percent from last year in the advertiser-coveted adults 18-49 demo, according to Nielsen.

“Idol’s” main competition was CBS’ “NCIS,” which held its own, averaging 16 million viewers at 8 p.m.

ABC, aware of the “Idol” steamroller, aired a “Dancing With the Stars” repeat against “Idol” at 8 p.m., then segued into its live finale show (9-11 p.m.).

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