diff --git a/airflow-core/src/airflow/api_fastapi/execution_api/routes/task_instances.py b/airflow-core/src/airflow/api_fastapi/execution_api/routes/task_instances.py
index 0fccd6ad1b61c..7740b6c72f45d 100644
--- a/airflow-core/src/airflow/api_fastapi/execution_api/routes/task_instances.py
+++ b/airflow-core/src/airflow/api_fastapi/execution_api/routes/task_instances.py
@@ -321,8 +321,9 @@ def ti_run(
     "/{task_instance_id}/state",
     status_code=status.HTTP_204_NO_CONTENT,
     responses={
+        status.HTTP_200_OK: {"description": "The TI was already in the requested state"},
         status.HTTP_404_NOT_FOUND: {"description": "Task Instance not found"},
-        status.HTTP_409_CONFLICT: {"description": "The TI is already in the requested state"},
+        status.HTTP_409_CONFLICT: {"description": "The TI is not in a valid state for this transition"},
         HTTP_422_UNPROCESSABLE_CONTENT: {"description": "Invalid payload for the state transition"},
     },
 )
@@ -389,6 +390,18 @@ def ti_update_state(
             },
         )
 
+    # TIStateUpdate can include terminal and intermediate states. This idempotency check handles
+    # duplicate updates when the requested state is already persisted (for example SUCCESS ->
+    # SUCCESS or DEFERRED -> DEFERRED), including duplicates that would not pass the RUNNING
+    # transition check below.
+    if ti_patch_payload.state.value == previous_state:
+        log.info(
+            "Duplicate state update request received; state already set",
+            requested_state=ti_patch_payload.state.value,
+            previous_state=previous_state,
+        )
+        return Response(status_code=status.HTTP_200_OK)
+
     if previous_state != TaskInstanceState.RUNNING:
         log.warning(
             "Cannot update Task Instance in invalid state",
diff --git a/airflow-core/tests/unit/api_fastapi/execution_api/versions/head/test_task_instances.py b/airflow-core/tests/unit/api_fastapi/execution_api/versions/head/test_task_instances.py
index 94b3fed0e071b..62c221dd0e933 100644
--- a/airflow-core/tests/unit/api_fastapi/execution_api/versions/head/test_task_instances.py
+++ b/airflow-core/tests/unit/api_fastapi/execution_api/versions/head/test_task_instances.py
@@ -1826,6 +1826,62 @@ def test_ti_update_state_not_running(self, client, session, create_task_instance
         session.refresh(ti)
         assert ti.state == State.SUCCESS
 
+    def test_ti_update_state_same_state_is_idempotent(self, client, session, create_task_instance):
+        """Test that setting a TI to its current state is treated as an idempotent no-op."""
+        ti = create_task_instance(
+            task_id="test_ti_update_state_same_state_is_idempotent",
+            state=State.SUCCESS,
+            session=session,
+            start_date=DEFAULT_START_DATE,
+        )
+        ti.end_date = DEFAULT_END_DATE
+        session.commit()
+
+        response = client.patch(
+            f"/execution/task-instances/{ti.id}/state",
+            json={
+                "state": "success",
+                "end_date": timezone.parse("2024-10-31T13:00:00Z").isoformat(),
+            },
+        )
+        assert response.status_code == 200
+        assert response.content == b""
+
+        session.refresh(ti)
+        assert ti.state == State.SUCCESS
+        assert ti.end_date == DEFAULT_END_DATE
+
+    def test_ti_update_state_terminal_state_mismatch_returns_conflict(
+        self, client, session, create_task_instance
+    ):
+        """A completed TI cannot be updated to a different state."""
+        ti = create_task_instance(
+            task_id="test_ti_update_state_terminal_state_mismatch_returns_conflict",
+            state=State.SUCCESS,
+            session=session,
+            start_date=DEFAULT_START_DATE,
+        )
+        ti.end_date = DEFAULT_END_DATE
+        session.commit()
+
+        response = client.patch(
+            f"/execution/task-instances/{ti.id}/state",
+            json={
+                "state": "failed",
+                "end_date": timezone.parse("2024-10-31T13:00:00Z").isoformat(),
+            },
+        )
+        assert response.status_code == 409
+        assert response.json()["detail"] == {
+            "reason": "invalid_state",
+            "message": "TI was not in the running state so it cannot be updated",
+            "previous_state": State.SUCCESS,
+        }
+
+        session.refresh(ti)
+        assert ti.state == State.SUCCESS
+        assert ti.end_date == DEFAULT_END_DATE
+
     def test_ti_update_state_to_failed_without_fail_fast(self, client, session, dag_maker):
         """Test that SerializedDAG is NOT loaded when fail_fast=False (default)."""
         with dag_maker(dag_id="test_dag_no_fail_fast", serialized=True):