feat(validation): refuse a variable declared by two input specs

tests/inputs/test_inputs.yml

@@ -14,6 +14,26 @@
         loop_var: inf
         label: "{{ inf.name }}"
 
+    - name: Refuse a variable declared by two specs
+      block:
+        - name: Run the validator with the fixture spec twice
+          validate_inputs:
+            specs:
+              - "{{ playbook_dir }}/fixture_spec.yml"
+              - "{{ playbook_dir }}/fixture_spec.yml"
+          register: inputs_duplicate_result
+      rescue:
+        - name: Keep the duplicate-declaration failure
+          ansible.builtin.set_fact:
+            inputs_duplicate_result: "{{ ansible_failed_result }}"
+
+    - name: Assert the duplicate declaration was refused
+      ansible.builtin.assert:
+        that:
+          - inputs_duplicate_result.msg is search('fixture is declared by both')
+        fail_msg: "[inputs duplicate] got {{ inputs_duplicate_result.msg | default('') }}"
+        success_msg: "[inputs duplicate] OK"
+
     - name: Validate a host var holding values that cannot be decrypted
       block:
         - name: Run the validator on the vaulted host var