They copied Joker's insanity and put it in the Goblin.The maniacal way of thinking is pretty much the same between the two,the weapons they use have a similarity between them.They even copied Joker's maniacal laughter,the relationship between Spider-Man and the Goblin is very similar to the relationship between Batman and the Joker,except the fact that Joker doesn't have a son/daughter to be friends with Batman.
Ummm, the Green Goblin isn't actually crazy. In the comics at least, it's just an act. The Green Goblin is a secret identity, while the Joker is jsut himself. Also, how exactly do they have the same weapons? Does the joker have a flying contraption and pumpkin bombs? I don't even know of any trademark weapons per se that the Joker has (unless you count nerve gas, and the Green Goblin doesn't use that if I'm not mistaken).
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