diff --git a/libs/langgraph/langgraph/graph/message.py b/libs/langgraph/langgraph/graph/message.py
index ed6322b5b..60810bc41 100644
--- a/libs/langgraph/langgraph/graph/message.py
+++ b/libs/langgraph/langgraph/graph/message.py
@@ -176,9 +176,12 @@ def add_messages(
     for m in left:
         if m.id is None:
             m.id = str(uuid.uuid4())
+    dedupe_map = {}
     for m in right:
         if m.id is None:
             m.id = str(uuid.uuid4())
+        dedupe_map[m.id] = m
+    right = list(dedupe_map.values())
     # merge
     left_idx_by_id = {m.id: i for i, m in enumerate(left)}
     merged = left.copy()
diff --git a/libs/langgraph/tests/test_messages_state.py b/libs/langgraph/tests/test_messages_state.py
index 787774baf..461a8b336 100644
--- a/libs/langgraph/tests/test_messages_state.py
+++ b/libs/langgraph/tests/test_messages_state.py
@@ -66,6 +66,15 @@ def test_missing_ids():
     assert all(isinstance(m.id, str) and UUID(m.id, version=4) for m in result)
 
 
+def test_duplicates_in_input():
+    left = []
+    right = [AIMessage(id="1", content="Hi there!"), AIMessage(id="1", content="Hi there again!")]
+    result = add_messages(left, right)
+    assert len(result) == 1
+    assert result[0].id == "1"
+    assert result[0].content == "Hi there again!"
+
+
 def test_remove_message():
     left = [
         HumanMessage(content="Hello", id="1"),