Ansible-lint should warn about multiple registration of the same variable

[d-mankowski-synerise]

Consider

- name: Example
  hosts: localhost
  become: false
  gather_facts: false
  tasks:
    - name: Set fact about _cloud
      ansible.builtin.set_fact:
        _cloud: azure

    - name: Content is foo
      ansible.builtin.command: "echo foo"
      register: _var
      when: _cloud == "azure"

    - name: Debug _var
      ansible.builtin.debug:
        var: _var.stdout

    - name: Content is bar
      ansible.builtin.command: "echo bar"
      register: _var
      when: _cloud == "aws"

    - name: Debug _var
      ansible.builtin.debug:
        var: _var.stdout

One would expect, that since _cloud is azure, then the second ansible.builtin.command is not executed, just like second register. However, that is not the case:

TASK [Set fact about _cloud] ************************************************************************************************************************************
Tuesday 23 September 2025  15:03:25 +0200 (0:00:00.031)       0:00:00.031 ***** 
Tuesday 23 September 2025  15:03:25 +0200 (0:00:00.030)       0:00:00.030 ***** 
ok: [localhost]

TASK [Content is foo] *******************************************************************************************************************************************
Tuesday 23 September 2025  15:03:25 +0200 (0:00:00.014)       0:00:00.046 ***** 
Tuesday 23 September 2025  15:03:25 +0200 (0:00:00.014)       0:00:00.045 ***** 
changed: [localhost]

TASK [Debug _var] ***********************************************************************************************************************************************
Tuesday 23 September 2025  15:03:26 +0200 (0:00:00.749)       0:00:00.796 ***** 
Tuesday 23 September 2025  15:03:26 +0200 (0:00:00.749)       0:00:00.795 ***** 
ok: [localhost] => {
    "_var.stdout": "foo"
}

TASK [Content is bar] *******************************************************************************************************************************************
Tuesday 23 September 2025  15:03:26 +0200 (0:00:00.014)       0:00:00.810 ***** 
Tuesday 23 September 2025  15:03:26 +0200 (0:00:00.014)       0:00:00.809 ***** 
skipping: [localhost]

TASK [Debug _var] ***********************************************************************************************************************************************
Tuesday 23 September 2025  15:03:26 +0200 (0:00:00.011)       0:00:00.821 ***** 
Tuesday 23 September 2025  15:03:26 +0200 (0:00:00.011)       0:00:00.820 ***** 
ok: [localhost] => {
    "_var.stdout": "VARIABLE IS NOT DEFINED!"
}

As you can see, even though task Content is bar was skipped, the value of _var was changed. It is very easy to miss that, and debugging is usually really annoying, if someone is not aware of this behavior. IMO anisble-lint should report if the same variable is registered more than once.