diff --git a/libs/langgraph/langgraph/graph/message.py b/libs/langgraph/langgraph/graph/message.py
index ed6322b5b..b6ed16131 100644
--- a/libs/langgraph/langgraph/graph/message.py
+++ b/libs/langgraph/langgraph/graph/message.py
@@ -180,14 +180,15 @@ def add_messages(
         if m.id is None:
             m.id = str(uuid.uuid4())
     # merge
-    left_idx_by_id = {m.id: i for i, m in enumerate(left)}
     merged = left.copy()
+    merged_by_id = {m.id: i for i, m in enumerate(merged)}
     ids_to_remove = set()
     for m in right:
-        if (existing_idx := left_idx_by_id.get(m.id)) is not None:
+        if (existing_idx := merged_by_id.get(m.id)) is not None:
             if isinstance(m, RemoveMessage):
                 ids_to_remove.add(m.id)
             else:
+                ids_to_remove.discard(m.id)
                 merged[existing_idx] = m
         else:
             if isinstance(m, RemoveMessage):
@@ -195,6 +196,7 @@ def add_messages(
                     f"Attempting to delete a message with an ID that doesn't exist ('{m.id}')"
                 )
 
+            merged_by_id[m.id] = len(merged)
             merged.append(m)
     merged = [m for m in merged if m.id not in ids_to_remove]
 
diff --git a/libs/langgraph/tests/test_messages_state.py b/libs/langgraph/tests/test_messages_state.py
index 787774baf..97f38051c 100644
--- a/libs/langgraph/tests/test_messages_state.py
+++ b/libs/langgraph/tests/test_messages_state.py
@@ -66,6 +66,31 @@ 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_duplicates_in_input_with_remove():
+    left = [AIMessage(id="1", content="Hello!")]
+    right = [
+        RemoveMessage(id="1"),
+        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"),