for The Trouble with Mark Hopper by Elissa Brent Weissman
Mark Hopper is the smartest student in the sixth grade, and he knows it. When another Mark Hopper moves to town (the same age, with similar looks), Mark Hopper makes trouble for Mark Hopper. The Mark everyone knows isn't very nice, so the other Mark finds no one will talk to him. The new Mark isn't very brainy, so the other Mark worries that people will think he's a dummy. When a teacher forces them to be study partners, they realize that they can benefit from one another's strengths - and maybe win the Mastermind Challenge together. Trouble is, one of the Marks wants to win so badly that he's willing to steal from Mark.
Publisher description retrieved from Google Books.