diff --git a/litellm/litellm_core_utils/realtime_streaming.py b/litellm/litellm_core_utils/realtime_streaming.py
index 2c29c454101..1d3528832df 100644
--- a/litellm/litellm_core_utils/realtime_streaming.py
+++ b/litellm/litellm_core_utils/realtime_streaming.py
@@ -917,33 +917,40 @@ class RealTimeStreaming:
                     # and bypass the transcription guardrail after the
                     # initial disable. Covers both the flat beta key and the
                     # nested GA ``audio.input.turn_detection`` shape, since
-                    # the GA remap below also accepts either form.
+                    # the GA remap below also accepts either form. Skipped
+                    # when the injection block above already ran for this
+                    # message, to avoid redundant double-serialization.
                     if (
                         msg_type == "session.update"
+                        and not guardrail_turn_detection_injected
                         and self._has_audio_transcription_guardrails()
                     ):
                         session = msg_obj.get("session")
                         if isinstance(session, dict):
                             td_overridden = False
                             flat_td = session.get("turn_detection")
-                            if (
-                                isinstance(flat_td, dict)
-                                and flat_td.get("create_response") is not False
-                            ):
-                                flat_td["create_response"] = False
-                                td_overridden = True
+                            if flat_td is not None:
+                                if not isinstance(flat_td, dict):
+                                    flat_td = {}
+                                if flat_td.get("create_response") is not False:
+                                    flat_td["create_response"] = False
+                                    session["turn_detection"] = flat_td
+                                    td_overridden = True
                             audio = session.get("audio")
                             if isinstance(audio, dict):
                                 audio_input = audio.get("input")
                                 if isinstance(audio_input, dict):
                                     nested_td = audio_input.get("turn_detection")
-                                    if (
-                                        isinstance(nested_td, dict)
-                                        and nested_td.get("create_response")
-                                        is not False
-                                    ):
-                                        nested_td["create_response"] = False
-                                        td_overridden = True
+                                    if nested_td is not None:
+                                        if not isinstance(nested_td, dict):
+                                            nested_td = {}
+                                        if (
+                                            nested_td.get("create_response")
+                                            is not False
+                                        ):
+                                            nested_td["create_response"] = False
+                                            audio_input["turn_detection"] = nested_td
+                                            td_overridden = True
                             if td_overridden:
                                 message = json.dumps(msg_obj)