Originally discussed on Matrix.

TLDR; Ansible handlers are added to the global namespace.

Suppose you've got a role which defines a handler MyHandler:

- name: MyHandler
  ...
  listen: "some-topic"

Each time you import/include your role, a new reference to MyHandler is added to the global namespace.

As a result, when you notify your handler via the topics it listens to (ie notify: "some-topic"), all the references to MyHandler will be executed by Ansible.

If that's not what you want, you should notify the handler by name (ie notify: MyHandler) in which case Ansible will stop searching for other references as soon as it finds the first occurrence of MyHandler. That means MyHandler will be executed only once.