Consider the following content type definitions :

• Mammal with a global validator hasThreeEarBones
• Duck extends Mammal
• Beaver extends Mammal

Now consider a content Platypus of content type Duck and Beaver.

When editing the Platypus content, the hasThreeEarBones will be called twice (once for each type including it) even though it is in fact the same validator.
Aside from the obvious time loss, this also means that in case of error detected by the validator, those error will be detected multiple time and display multiple time to the user.