In "Moaning Lisa," Bart and Homer get involved in video-game boxing. The boxers displayed in the game look very similar to Homer and Bart. Production-wise this makes sense, as it eliminates the need to explain which character is controlling which boxer, etc. But is there a canon explanation why there is a similarity? Or is it a coincidence?
Wikipedia, citing the episode's DVD commentary, states that
The designs of the boxers in the video game Homer and Bart play were loosely based on Homer and Bart
So from a production standpoint, the resemblance was deliberate, and presumably done so that the viewers would know who was controlling which character. In-universe, though, it was almost certainly coincidence.
There's another, more prominent example of these intentional design similarities: Matt Groening designed Krusty the Clown to resemble Homer, to create a "satirical conceit" in which Bart "had no respect for his father, but worshipped a clown who looked exactly like his father". Yet in-universe, the resemblance between Homer and Krusty is pure coincidence. In-universe, the resemblance between them forms part of the plot of the episode "Homie the Clown", but is otherwise coincidental.