community.general/test/integration/targets/handlers/test_handlers_template_run_once.yml

- hosts: A,B
  gather_facts: no
  tasks:
    - debug:
      changed_when: true
      notify:
        - handler
  handlers:
    - name: handler
      debug:
        msg: "handler {{ inventory_hostname }}"
      run_once: "{{ testvar | default(False) }}"