Everyone knows that imitation is the sincerest form of flattery, and Kelly Clarkson flattered fellow 'American Idol' winner Carrie Underwood on Wednesday night when she performed her song 'I Know You Won't' during a free concert in Los Angeles.

Sandwiched between her own slew of hits -- like 'Since U Been Gone,' 'Miss Independent,' 'Sober' and 'Breakaway,' to name a few -- Clarkson nailed down the notes on Underwood's track from her second studio release, 2007's 'Carnival Ride.' The show was an intimate event meant to help the first 'Idol' winner promote her forthcoming album, 'Stronger,' which impacts on October 24.

While Underwood did not make a surprise appearance at Clarkson's show at the Troubadour, she did shock audiences at the Grand Ole Opry Brad Paisley concert on Tuesday night when she came out to perform 'Remind Me' with him for the first time ever live.

Watch Kelly Clarkson Perform Carrie Underwood's 'I Know You Won't'