diff --git a/litellm/llms/cohere/embed/handler.py b/litellm/llms/cohere/embed/handler.py
index 41b81279723..3ab8baf7ba8 100644
--- a/litellm/llms/cohere/embed/handler.py
+++ b/litellm/llms/cohere/embed/handler.py
@@ -21,14 +21,18 @@ from .v1_transformation import CohereEmbeddingConfig
 
 
 def validate_environment(api_key, headers: dict):
-    headers.update(
-        {
-            "Request-Source": "unspecified:litellm",
-            "accept": "application/json",
-            "content-type": "application/json",
-        }
-    )
-    if api_key:
+    # Create a lowercase key lookup to avoid duplicate headers with different cases
+    # This is important when headers come from AWS signed requests (which use Title-Case)
+    existing_keys_lower = {k.lower(): k for k in headers.keys()}
+
+    # Only add headers if they don't already exist (case-insensitive check)
+    if "request-source" not in existing_keys_lower:
+        headers["Request-Source"] = "unspecified:litellm"
+    if "accept" not in existing_keys_lower:
+        headers["accept"] = "application/json"
+    if "content-type" not in existing_keys_lower:
+        headers["content-type"] = "application/json"
+    if api_key and "authorization" not in existing_keys_lower:
         headers["Authorization"] = f"Bearer {api_key}"
     return headers