Does Kelly Clarkson remind people sometimes of Katy? - I wouldn't suggest that I'd confuse her with Katy but often when she sings those notes changing from chest to throat and back I find the sound gets so Katy-like.
It doesn't have to be one of the songs which Katy wrote and of which Katy's unreleased versions exist but could just be anything, and when I listen to some of Kelly's songs I find she sounds from nothing at all like Katy to suddenly very much like Katy - and back to not at all,haha, so it's not a simple 'she sounds like Katy' thing but it depends on which notes she sings and how.
Kelly Clarkson and Katy Perry both have extremely versatile voices that can sing just about anything. They are definitely two of the best female singers of the current generation. I don't normally think that they sound alike, but since their voices are so similar, they definitely can sound like each other.
Interestingly, although usually nobody in my office listens to music at work, one day last year, this guy did have the radio on (he's famous for his lack of music knowledge, among other things). When Kelly Clarkson's "A Moment Like This" came on, another coworker asked him who sang it. He erroneously said "Katy Perry". lol. Later that day, "Roar" came on. That same coworker asked him who sings it, and he said "I think Katy Perry".