Duck typing is a superset of inheritance. If your language only supports polymorphism via inheritance, then it is strictly less expressive than a language with duck typing.
What does superset mean, here? Inheritance can mandate the presence of certain fields at compile time, which duck typing cannot do, so it doesn't fit my traditional definition of "superset".
Some forms of duck typing absolutely can do this. "Row polymorphism" is the general feature that allows this, some duck typing can handle this, some can't.
