diff --git a/litellm/llms/anthropic/chat/transformation.py b/litellm/llms/anthropic/chat/transformation.py
index 64d885767cf..dc3100f4670 100644
--- a/litellm/llms/anthropic/chat/transformation.py
+++ b/litellm/llms/anthropic/chat/transformation.py
@@ -192,6 +192,13 @@ def _build_anthropic_tool_name_maps(
         candidate = _basic_sanitize_anthropic_tool_name(original)
         if candidate == original:
             continue
+        # Skip duplicates of the same original name. Without this guard the
+        # second pass would assign a fresh suffix and overwrite the forward
+        # map entry, causing every reference to map to the suffixed name and
+        # leaving the original sanitized slot orphaned in `used` with no
+        # reverse mapping.
+        if original in forward:
+            continue
         # Disambiguate against names already chosen this request.
         unique = candidate
         n = 1
diff --git a/tests/test_litellm/llms/anthropic/chat/test_anthropic_chat_transformation.py b/tests/test_litellm/llms/anthropic/chat/test_anthropic_chat_transformation.py
index 7054c515017..6f67d5417d2 100644
--- a/tests/test_litellm/llms/anthropic/chat/test_anthropic_chat_transformation.py
+++ b/tests/test_litellm/llms/anthropic/chat/test_anthropic_chat_transformation.py
@@ -4043,6 +4043,26 @@ def test_build_anthropic_tool_name_maps_reverse_order_collision():
     assert "foo_bar" not in reverse
 
 
+def test_build_anthropic_tool_name_maps_duplicate_originals():
+    """REGRESSION: duplicate originals must not corrupt the forward map.
+
+    Previously, the second occurrence of the same invalid name would
+    rewrite ``forward[original]`` to a suffixed name (``foo_bar_2``),
+    leaving ``foo_bar`` orphaned in ``used`` with no reverse mapping —
+    so when ``_sanitize_tool_names_in_request`` applied the forward
+    map, *both* tool entries got the suffixed name and Anthropic 400'd
+    on duplicates.
+    """
+    from litellm.llms.anthropic.chat.transformation import (
+        _build_anthropic_tool_name_maps,
+    )
+
+    forward, reverse = _build_anthropic_tool_name_maps(["foo/bar", "foo/bar"])
+    # Same original sanitizes to the same target — no spurious suffix.
+    assert forward == {"foo/bar": "foo_bar"}
+    assert reverse == {"foo_bar": "foo/bar"}
+
+
 def test_map_openai_params_does_not_pollute_optional_params_with_internal_keys():
     """REGRESSION: ``optional_params`` is what becomes the JSON body sent to
     Anthropic (``data = {**optional_params}``). It MUST NOT carry LiteLLM-