diff --git a/tests/tools/test_todo_tool.py b/tests/tools/test_todo_tool.py
index d4fd03baf..621507852 100644
--- a/tests/tools/test_todo_tool.py
+++ b/tests/tools/test_todo_tool.py
@@ -24,6 +24,18 @@ class TestWriteAndRead:
         items[0]["content"] = "MUTATED"
         assert store.read()[0]["content"] == "Task"
 
+    def test_write_deduplicates_duplicate_ids(self):
+        store = TodoStore()
+        result = store.write([
+            {"id": "1", "content": "First version", "status": "pending"},
+            {"id": "2", "content": "Other task", "status": "pending"},
+            {"id": "1", "content": "Latest version", "status": "in_progress"},
+        ])
+        assert result == [
+            {"id": "2", "content": "Other task", "status": "pending"},
+            {"id": "1", "content": "Latest version", "status": "in_progress"},
+        ]
+
 
 class TestHasItems:
     def test_empty_store(self):
diff --git a/tools/todo_tool.py b/tools/todo_tool.py
index 9021fbc2d..b0d38a234 100644
--- a/tools/todo_tool.py
+++ b/tools/todo_tool.py
@@ -46,11 +46,11 @@ class TodoStore:
         """
         if not merge:
             # Replace mode: new list entirely
-            self._items = [self._validate(t) for t in todos]
+            self._items = [self._validate(t) for t in self._dedupe_by_id(todos)]
         else:
             # Merge mode: update existing items by id, append new ones
             existing = {item["id"]: item for item in self._items}
-            for t in todos:
+            for t in self._dedupe_by_id(todos):
                 item_id = str(t.get("id", "")).strip()
                 if not item_id:
                     continue  # Can't merge without an id
@@ -143,6 +143,15 @@ class TodoStore:
 
         return {"id": item_id, "content": content, "status": status}
 
+    @staticmethod
+    def _dedupe_by_id(todos: List[Dict[str, Any]]) -> List[Dict[str, Any]]:
+        """Collapse duplicate ids, keeping the last occurrence in its position."""
+        last_index: Dict[str, int] = {}
+        for i, item in enumerate(todos):
+            item_id = str(item.get("id", "")).strip() or "?"
+            last_index[item_id] = i
+        return [todos[i] for i in sorted(last_index.values())]
+
 
 def todo_tool(
     todos: Optional[List[Dict[str, Any]]] = None,