Lennie is older than Duddy, and trying to become successful as a medical student. Their father likes Lennie more, giving Duddy more and more reason to try and impress him. When he is first introduced, it is clear that Lennie takes his studies very seriously, and wants to succeed both as a doctor and as a member of the upper class:
Duddy didn't get home until after seven o'clock. His father was out, but he found Lennie in the bedroom.
"Duddy," Lennie said, "how many times have I asked you not to barge in here when I'm studying?"
Duddy's face flushed.
(Richler, The Apprenticeship of Duddy Kravitz, Google Books)
Duddy, in comparison, doesn't want to have to work very hard to succeed, and continuously tries to succeed with shortcuts and illegal tricks. Ironically, it is Duddy's skill with people that saves Lennie from the shame of failing an illegal abortion. Both boys want to succeed to impress their father, but their methods are different, as are their ultimate goals. The major difference between them is in the way they approach relationships; Lennie wants to be accepted and falls prey to predatory friends who only want to use him for their own gains, while Duddy is successfully able to leverage some of those same people for his own ends while keeping himself mostly safe from consequences.