Duck typing example in GDScript. An adaptation of: https://en.wikipedia.org/wiki/Duck_typing#In_Python

duck_typing.gd

class Duck extends Reference:
    func quack():
        print("Quack, quack!")

    func fly():
        print("Flap, Flap!")

class Person extends Reference:
    func quack():
        print("I'm Quackin'!")

    func fly():
        print("I'm Flyin'!")

func in_the_forest(mallard):
    mallard.quack()
    mallard.fly()

# Constructor
func _init():
    in_the_forest(Duck.new())
    in_the_forest(Person.new())

# Output:
# Quack, quack!
# Flap, Flap!
# I'm Quackin'!
# I'm Flyin'!

# Notes:
# Every file in GDScript is a class and there is no global scope nor main function, therefore our entry point in this example will be the constructor
# In this example, Duck and Person are nested classes and they extend Reference to make code clearer
# Otherwise, if extending Object (or nothing which is equivalent to extending Object) you would need to manually free them by calling .free()