diff --git a/src/_shims/severless_wsgi.py b/src/_shims/severless_wsgi.py
index 2de91a3..ba18d74 100644
--- a/src/_shims/severless_wsgi.py
+++ b/src/_shims/severless_wsgi.py
@@ -55,11 +55,10 @@ def split_headers(headers):
     """
     new_headers = {}
 
-    for key in headers.keys():
+    for key in set(headers.keys()):
         values = headers.get_all(key)
         if len(values) > 1:
-            for value, casing in zip(values, all_casings(key)):
-                new_headers[casing] = value
+            new_headers[key] = values
         elif len(values) == 1:
             new_headers[key] = values[0]