diff --git a/src/openai/lib/streaming/_assistants.py b/src/openai/lib/streaming/_assistants.py
index 6efb3ca3f1..4ab97c8cf0 100644
--- a/src/openai/lib/streaming/_assistants.py
+++ b/src/openai/lib/streaming/_assistants.py
@@ -980,11 +980,19 @@ def accumulate_event(
 def accumulate_delta(acc: dict[object, object], delta: dict[object, object]) -> dict[object, object]:
     for key, delta_value in delta.items():
         if key not in acc:
+            if is_list(delta_value):
+                acc[key] = _accumulate_list_delta([], delta_value)
+                continue
+
             acc[key] = delta_value
             continue
 
         acc_value = acc[key]
         if acc_value is None:
+            if is_list(delta_value):
+                acc[key] = _accumulate_list_delta([], delta_value)
+                continue
+
             acc[key] = delta_value
             continue
 
@@ -1005,34 +1013,44 @@ def accumulate_delta(acc: dict[object, object], delta: dict[object, object]) ->
         elif is_dict(acc_value) and is_dict(delta_value):
             acc_value = accumulate_delta(acc_value, delta_value)
         elif is_list(acc_value) and is_list(delta_value):
-            # for lists of non-dictionary items we'll only ever get new entries
-            # in the array, existing entries will never be changed
-            if all(isinstance(x, (str, int, float)) for x in acc_value):
-                acc_value.extend(delta_value)
-                continue
+            acc_value = _accumulate_list_delta(acc_value, delta_value)
 
-            for delta_entry in delta_value:
-                if not is_dict(delta_entry):
-                    raise TypeError(f"Unexpected list delta entry is not a dictionary: {delta_entry}")
+        acc[key] = acc_value
 
-                try:
-                    index = delta_entry["index"]
-                except KeyError as exc:
-                    raise RuntimeError(f"Expected list delta entry to have an `index` key; {delta_entry}") from exc
+    return acc
 
-                if not isinstance(index, int):
-                    raise TypeError(f"Unexpected, list delta entry `index` value is not an integer; {index}")
 
-                try:
-                    acc_entry = acc_value[index]
-                except IndexError:
-                    acc_value.insert(index, delta_entry)
-                else:
-                    if not is_dict(acc_entry):
-                        raise TypeError("not handled yet")
+def _accumulate_list_delta(acc: list[object], delta: list[object]) -> list[object]:
+    # for lists of non-dictionary items we'll only ever get new entries
+    # in the array, existing entries will never be changed
+    if all(isinstance(x, (str, int, float)) for x in [*acc, *delta]):
+        acc.extend(delta)
+        return acc
 
-                    acc_value[index] = accumulate_delta(acc_entry, delta_entry)
+    for delta_entry in delta:
+        if not is_dict(delta_entry):
+            raise TypeError(f"Unexpected list delta entry is not a dictionary: {delta_entry}")
 
-        acc[key] = acc_value
+        try:
+            index = delta_entry["index"]
+        except KeyError as exc:
+            raise RuntimeError(f"Expected list delta entry to have an `index` key; {delta_entry}") from exc
+
+        if not isinstance(index, int):
+            raise TypeError(f"Unexpected, list delta entry `index` value is not an integer; {index}")
+
+        acc_entry = None
+        acc_index = None
+        for i, entry in enumerate(acc):
+            if is_dict(entry) and entry.get("index") == index:
+                acc_entry = entry
+                acc_index = i
+                break
+
+        if acc_entry is None:
+            acc.insert(index, delta_entry)
+        else:
+            assert acc_index is not None
+            acc[acc_index] = accumulate_delta(acc_entry, delta_entry)
 
     return acc
diff --git a/src/openai/lib/streaming/_deltas.py b/src/openai/lib/streaming/_deltas.py
index a5e1317612..35a97a0b55 100644
--- a/src/openai/lib/streaming/_deltas.py
+++ b/src/openai/lib/streaming/_deltas.py
@@ -6,11 +6,19 @@
 def accumulate_delta(acc: dict[object, object], delta: dict[object, object]) -> dict[object, object]:
     for key, delta_value in delta.items():
         if key not in acc:
+            if is_list(delta_value):
+                acc[key] = _accumulate_list_delta([], delta_value)
+                continue
+
             acc[key] = delta_value
             continue
 
         acc_value = acc[key]
         if acc_value is None:
+            if is_list(delta_value):
+                acc[key] = _accumulate_list_delta([], delta_value)
+                continue
+
             acc[key] = delta_value
             continue
 
@@ -31,34 +39,44 @@ def accumulate_delta(acc: dict[object, object], delta: dict[object, object]) ->
         elif is_dict(acc_value) and is_dict(delta_value):
             acc_value = accumulate_delta(acc_value, delta_value)
         elif is_list(acc_value) and is_list(delta_value):
-            # for lists of non-dictionary items we'll only ever get new entries
-            # in the array, existing entries will never be changed
-            if all(isinstance(x, (str, int, float)) for x in acc_value):
-                acc_value.extend(delta_value)
-                continue
+            acc_value = _accumulate_list_delta(acc_value, delta_value)
 
-            for delta_entry in delta_value:
-                if not is_dict(delta_entry):
-                    raise TypeError(f"Unexpected list delta entry is not a dictionary: {delta_entry}")
+        acc[key] = acc_value
 
-                try:
-                    index = delta_entry["index"]
-                except KeyError as exc:
-                    raise RuntimeError(f"Expected list delta entry to have an `index` key; {delta_entry}") from exc
+    return acc
 
-                if not isinstance(index, int):
-                    raise TypeError(f"Unexpected, list delta entry `index` value is not an integer; {index}")
 
-                try:
-                    acc_entry = acc_value[index]
-                except IndexError:
-                    acc_value.insert(index, delta_entry)
-                else:
-                    if not is_dict(acc_entry):
-                        raise TypeError("not handled yet")
+def _accumulate_list_delta(acc: list[object], delta: list[object]) -> list[object]:
+    # for lists of non-dictionary items we'll only ever get new entries
+    # in the array, existing entries will never be changed
+    if all(isinstance(x, (str, int, float)) for x in [*acc, *delta]):
+        acc.extend(delta)
+        return acc
 
-                    acc_value[index] = accumulate_delta(acc_entry, delta_entry)
+    for delta_entry in delta:
+        if not is_dict(delta_entry):
+            raise TypeError(f"Unexpected list delta entry is not a dictionary: {delta_entry}")
 
-        acc[key] = acc_value
+        try:
+            index = delta_entry["index"]
+        except KeyError as exc:
+            raise RuntimeError(f"Expected list delta entry to have an `index` key; {delta_entry}") from exc
+
+        if not isinstance(index, int):
+            raise TypeError(f"Unexpected, list delta entry `index` value is not an integer; {index}")
+
+        acc_entry = None
+        acc_index = None
+        for i, entry in enumerate(acc):
+            if is_dict(entry) and entry.get("index") == index:
+                acc_entry = entry
+                acc_index = i
+                break
+
+        if acc_entry is None:
+            acc.insert(index, delta_entry)
+        else:
+            assert acc_index is not None
+            acc[acc_index] = accumulate_delta(acc_entry, delta_entry)
 
     return acc
diff --git a/tests/lib/test_streaming_deltas.py b/tests/lib/test_streaming_deltas.py
new file mode 100644
index 0000000000..5defed1235
--- /dev/null
+++ b/tests/lib/test_streaming_deltas.py
@@ -0,0 +1,37 @@
+from __future__ import annotations
+
+from collections.abc import Callable
+
+import pytest
+
+from openai.lib.streaming._deltas import accumulate_delta as accumulate_chat_delta
+from openai.lib.streaming._assistants import accumulate_delta as accumulate_assistants_delta
+
+
+@pytest.mark.parametrize("accumulate_delta", [accumulate_chat_delta, accumulate_assistants_delta])
+def test_accumulate_delta_merges_duplicate_index_entries_in_initial_list(
+    accumulate_delta: Callable[[dict[object, object], dict[object, object]], dict[object, object]],
+) -> None:
+    acc: dict[object, object] = {}
+
+    accumulate_delta(
+        acc,
+        {
+            "tool_calls": [
+                {"index": 0, "id": "call_abc", "function": {"name": "list_files"}, "type": "function"},
+                {"index": 0, "function": {"arguments": '{"path"'}},
+            ]
+        },
+    )
+    accumulate_delta(acc, {"tool_calls": [{"index": 0, "function": {"arguments": ': "."}'}}]})
+
+    assert acc == {
+        "tool_calls": [
+            {
+                "index": 0,
+                "id": "call_abc",
+                "function": {"name": "list_files", "arguments": '{"path": "."}'},
+                "type": "function",
+            }
+        ]
+    }