Kelly Clarkson once signed an autograph as Carrie Underwood

Kelly Clarkson was once mistaken for fellow American Idol winner Carrie Underwood - and even signed an autograph as the country music star.

Speaking on her The Kelly Clarkson Show, the singer and her guests, Michelle Buteau and Cheryl Hines, opened up about awkward moments in their lives as the Breakaway hitmaker discussed the celeb mix up.

"This girl came up to me and was like, 'Oh my god, your song So Small is one of my favourites'," recalled Kelly. "And I kept thinking, 'I don't have a song So Small.' And then she said another song and I was like, 'Oh, you think I'm Carrie Underwood'."

Laughing that she looks "nothing like Carrie Underwood," the Grammy winner added: "I was so embarrassed for her that I just... She was like, 'Can you sign this?' And I totally signed 'Carrie Underwood'. I think that might be illegal!"

Kelly won the first season of the TV singing contest back in 2002 while Carrie triumphed on her season in 2005.