diff --git a/backend/app/modelling/router.py b/backend/app/modelling/router.py
index 71e74ae4f..6be284523 100644
--- a/backend/app/modelling/router.py
+++ b/backend/app/modelling/router.py
@@ -12,7 +12,7 @@ from collections.abc import Callable, Iterator
 from typing import Any, cast
 
 import boto3
-from fastapi import APIRouter, Depends
+from fastapi import APIRouter, Depends, HTTPException
 from sqlmodel import Session
 
 from backend.app.config import get_settings
@@ -70,6 +70,16 @@ async def trigger_run(
     session: Session = Depends(get_session),
     send_messages: MessageSender = Depends(get_message_sender),
 ) -> dict[str, str]:
+    subtask_repo = SubTaskPostgresRepository(session)
+    if subtask_repo.list_by_task(body.task_id):
+        raise HTTPException(
+            status_code=409,
+            detail=(
+                f"Task {body.task_id} already has sub_tasks — it has been "
+                "distributed. Check its progress instead of re-triggering."
+            ),
+        )
+
     properties: list[FilteredProperty] = resolve_filtered_property_ids(
         session, body.portfolio_id, body.filters
     )
@@ -78,7 +88,6 @@ async def trigger_run(
     # Pre-create one sub_task per (scenario, batch) message under the
     # app-owned task, each holding its exact message payload — the fixed
     # progress denominator and the batch's re-run recipe (ADR-0055).
-    subtask_repo = SubTaskPostgresRepository(session)
     subtasks: list[SubTask] = []
     payloads: list[dict[str, Any]] = []
     for scenario_id in body.scenario_ids: