Someone who has apparently had everyone elses experiences happen to them, but much better or much worse. Often this is because they need to be better than others or feel the need to be 'one-up' from everybody else.
Sophie: I got totally drunk and ended up falling asleep on a bench!
Jess: Yeah that happened to me but I woke up in a lion's cage and proper fought it off with my judo style skills. All with a hangover too!
Sophie: You are such a one-upper!