This Fowler and Beck [F] coding smell occurs when the interfaces of two classes are different and yet the classes are quite similar. If you can find the similarities between the two classes, you can often refactor the classes to make them share a common interface.
However, sometimes you can't directly change the interface of a class because you don't have control over the code. The typical example is when you're working with a third-party library. In that case, you can apply Unify Interfaces with Adapter (247) to produce a common interface for the two classes.