diff --git a/tests/backend/app/modelling/test_trigger_run_router.py b/tests/backend/app/modelling/test_trigger_run_router.py
index ae9e020be..d383fae1d 100644
--- a/tests/backend/app/modelling/test_trigger_run_router.py
+++ b/tests/backend/app/modelling/test_trigger_run_router.py
@@ -83,6 +83,37 @@ def api(db_engine: Engine) -> Api:
     return harness
 
 
+def _trigger(api: Api, task: Task, scenario_ids: list[int]) -> Any:
+    return api.client.post(
+        "/v1/modelling/trigger-run",
+        json={
+            "task_id": str(task.id),
+            "portfolio_id": PORTFOLIO_ID,
+            "scenario_ids": scenario_ids,
+            "filters": {},
+        },
+    )
+
+
+def test_trigger_run_rejects_a_task_that_already_has_subtasks(api: Api) -> None:
+    """A blind retry or double-submit must not double the fan-out (ADR-0055):
+    the app checks the task's progress instead of re-POSTing."""
+    # arrange — a task that has already been distributed
+    task = api.seed_task()
+    scenario = api.seed_scenario()
+    api.seed_property()
+    assert _trigger(api, task, [scenario]).status_code == 202
+    already_sent = len(api.sent_bodies)
+
+    # act
+    response = _trigger(api, task, [scenario])
+
+    # assert — refused, nothing new created or sent
+    assert response.status_code == 409
+    assert len(api.sent_bodies) == already_sent
+    assert len(api.subtasks_for(task)) == 1
+
+
 def test_trigger_run_fans_out_one_subtask_and_message_per_scenario_batch(
     api: Api,
 ) -> None: