diff --git a/app/repository/messages.py b/app/repository/messages.py
index 3c1f9269..6aaec093 100644
--- a/app/repository/messages.py
+++ b/app/repository/messages.py
@@ -180,8 +180,16 @@ class MessageRepository:
         """
         cursor = await db.conn.execute(
             """UPDATE messages SET sender_key = ?
-               WHERE type = 'CHAN' AND sender_name = ? AND sender_key IS NULL""",
-            (public_key.lower(), name),
+               WHERE type = 'CHAN' AND sender_name = ? AND sender_key IS NULL
+               AND (
+                   SELECT COUNT(*) FROM contacts
+                   WHERE name = ?
+               ) = 1
+               AND EXISTS (
+                   SELECT 1 FROM contacts
+                   WHERE public_key = ? AND name = ?
+               )""",
+            (public_key.lower(), name, name, public_key.lower(), name),
         )
         await db.conn.commit()
         return cursor.rowcount
diff --git a/tests/test_channel_sender_backfill.py b/tests/test_channel_sender_backfill.py
index 188ae2e8..057bea38 100644
--- a/tests/test_channel_sender_backfill.py
+++ b/tests/test_channel_sender_backfill.py
@@ -2,7 +2,7 @@
 
 import pytest
 
-from app.repository import MessageRepository
+from app.repository import ContactRepository, MessageRepository
 
 
 @pytest.mark.asyncio
@@ -36,6 +36,14 @@ async def test_backfill_sets_sender_key_on_matching_messages(test_db):
     assert all(m.sender_key is None for m in messages)
 
     # Contact becomes known
+    await ContactRepository.upsert(
+        {
+            "public_key": pub_key,
+            "name": "Alice",
+            "type": 1,
+            "flags": 0,
+        }
+    )
     backfilled = await MessageRepository.backfill_channel_sender_key(pub_key, "Alice")
     assert backfilled == 2
 
@@ -95,6 +103,14 @@ async def test_backfill_only_affects_matching_name(test_db):
         sender_name="Bob",
     )
 
+    await ContactRepository.upsert(
+        {
+            "public_key": pub_key,
+            "name": "Alice",
+            "type": 1,
+            "flags": 0,
+        }
+    )
     backfilled = await MessageRepository.backfill_channel_sender_key(pub_key, "Alice")
     assert backfilled == 1
 
@@ -138,8 +154,56 @@ async def test_backfill_idempotent(test_db):
         sender_name="Alice",
     )
 
+    await ContactRepository.upsert(
+        {
+            "public_key": pub_key,
+            "name": "Alice",
+            "type": 1,
+            "flags": 0,
+        }
+    )
     first = await MessageRepository.backfill_channel_sender_key(pub_key, "Alice")
     assert first == 1
 
     second = await MessageRepository.backfill_channel_sender_key(pub_key, "Alice")
     assert second == 0
+
+
+@pytest.mark.asyncio
+async def test_backfill_skips_ambiguous_duplicate_names(test_db):
+    """Duplicate contact names should leave channel sender_key unresolved."""
+    pub_key_a = "33" * 32
+    pub_key_b = "44" * 32
+    channel_key = "55" * 16
+
+    await ContactRepository.upsert(
+        {
+            "public_key": pub_key_a,
+            "name": "Alice",
+            "type": 1,
+            "flags": 0,
+        }
+    )
+    await ContactRepository.upsert(
+        {
+            "public_key": pub_key_b,
+            "name": "Alice",
+            "type": 1,
+            "flags": 0,
+        }
+    )
+    await MessageRepository.create(
+        msg_type="CHAN",
+        text="Alice: ambiguous",
+        conversation_key=channel_key,
+        sender_timestamp=100,
+        received_at=100,
+        sender_name="Alice",
+    )
+
+    backfilled = await MessageRepository.backfill_channel_sender_key(pub_key_a, "Alice")
+    assert backfilled == 0
+
+    messages = await MessageRepository.get_all(msg_type="CHAN", conversation_key=channel_key)
+    assert len(messages) == 1
+    assert messages[0].sender_key is None