Why Duck Typing Is Safe
For the last 20 years I've been programming almost exclusively in languages with either duck typing or structural typing. These are languages where you can have objects that either conform to an informal interface without that conformance being declared (dynamically-typed languages), or where conformance is simply a matter of matching the interface (Go). This is as opposed to languages where conformance must be explicitly declared somehow, either with a formal subclass relationship (the class-based portion of C++) or by explicitly declaring conformance to an interface (Java).
Not needing to formally declare conformance is powerful because you can take some pre-existing object, declare a new interface that it conforms to, and then use it with your interface. This reduces the endless paperwork around re-wrapping things to make existing objects conform to new interfaces.
I have seen a lot of programmers over the years express concerns; what if something accidentally conforms to an interface and causes some horrible bug? What if I have an interface for Shoot() for a video game and some object wanders in that Shoot()s a real gun by accident? Isn't it safer to formally declare conformance?