Python, unlike statically typed languages such as Java, allows complete freedom when calling methods during object destruction. However, standard object-oriented principles apply to Python classes using deep inheritance hierarchies. Therefore the developer has responsibility for ensuring that objects are properly cleaned up when there are multiple __del__ methods that need to be called.

Calling a __del__ method more than once during object destruction risks resources being released multiple times. The relevant __del__ method may not be designed to be called more than once.

There are a number of ways that a __del__ method may be be called more than once.

There may be more than one explicit call to the method in the hierarchy of __del__ methods.
A class using multiple inheritance directly calls the __del__ methods of its base types. One or more of those base types uses super() to pass down the inheritance chain.

Either be careful not to explicitly call a __del__ method more than once, or use super() throughout the inheritance hierarchy.

Alternatively refactor one or more of the classes to use composition rather than inheritance.

In the first example, explicit calls to __del__ are used, but SportsCar erroneously calls both Vehicle.__del__ and Car.__del__. This can be fixed by removing the call to Vehicle.__del__, as shown in FixedSportsCar.

In the second example, there is a mixture of explicit calls to __del__ and calls using super(). To fix this example, super() should be used throughout.

Python Tutorial: Classes.

Python Standard Library: super.

Artima Developer: Things to Know About Python Super.

Wikipedia: Composition over inheritance.