ansible/test/integration/targets/strategy_linear/task_templated_run_once.yml

- hosts: testhost,testhost2
  gather_facts: no
  vars:
    run_once_flag: false
  tasks:
    - debug:
        msg: "I am {{ item }}"
      run_once: "{{ run_once_flag }}"
      register: reg1
      loop:
        - "{{ inventory_hostname }}"

    - assert:
        that:
          - "reg1.results[0].msg == 'I am testhost'"
      when: inventory_hostname == 'testhost'
    - assert:
        that:
          - "reg1.results[0].msg == 'I am testhost2'"
      when: inventory_hostname == 'testhost2'