How to deal when you're an actual imposter in academia? I'm a second-year PhD student at a well-ranked university, and am seriously beginning to feel that I'm an imposter in this place. My lab group is filled with people who ranked in the top 0.01 percent of examinations in their home country, have won olympiads and international competitions, and published an insane amount while maintaining a 4.0 GPA during grad school, while here I am with a scrub 3.5 GPA in undergrad and no publications.

I honestly think that admissions screwed up somewhere with my application and that I really don't belong here. I really want to get a PhD and I love academic research, but feel that I just can't compete against such people. 

Has anyone ever gone through these feelings? I feel that I can't speak to anyone else about it because everyone seems so accomplished, and it'd only reinforce my feelings of inadequacy.
Own it. Channel these feelings into finding what makes you competitive and happy.

As an impostor that continues to 'make it', I have accepted that I bring something different. My undergrad GPA was 3.6, exactly, at a no-name school. I was accepted to a very good school for my PhD, and opted for a lower ranked school on the basis that I would 'have a better chance'. Likely, that was an error: I have since held a postdoc and now a faculty position at a high ranked school, with offers from other high-ranked schools. People puzzle over my CV, which is all kinds of nontraditional, but they also see evidence there that I do some things very well. I do, and these things make me competitive with the 4.0-olympiad-never-made-a-mistake crowd. Indeed, the fact that I continually make mistakes is itself a competitive edge; I'm inherently more fearless than my well-heeled peers.  This strategy has led to a career that some of my 'perfect' classmates have openly wondered at.  I wonder at it. I mean, I'm still a scrub, just a highly effective one. ;)

So, go do that.