diff --git a/packages/abstractions/kiota_abstractions/headers_collection.py b/packages/abstractions/kiota_abstractions/headers_collection.py
index 729295dc..0fa10363 100644
--- a/packages/abstractions/kiota_abstractions/headers_collection.py
+++ b/packages/abstractions/kiota_abstractions/headers_collection.py
@@ -98,19 +98,15 @@ def add(self, header_name: str, header_values: Union[str, list[str]]) -> None:
         if not header_values:  # empty list
             return
         header_name = header_name.lower()
-        if isinstance(header_values, list):
-            if header_name in self.SINGLE_VALUE_HEADERS:
-                self._headers[header_name] = {header_values[0]}
-            elif values := self.try_get(header_name):
-                for header_value in header_values:
-                    values.add(header_value)  #type: ignore
-            else:
-                self._headers[header_name] = set(header_values)
+        if not isinstance(header_values, list):
+            header_values = [header_values]
+        if header_name in self.SINGLE_VALUE_HEADERS:
+            self._headers[header_name] = {header_values[0]}
+        elif values := self.try_get(header_name):
+            for header_value in header_values:
+                values.add(header_value)  #type: ignore
         else:
-            if values := self.try_get(header_name):
-                values.add(header_values)  #type: ignore
-            else:
-                self._headers[header_name] = {header_values}
+            self._headers[header_name] = set(header_values)
 
     def keys(self) -> list[str]:
         """Gets the header names present in the collection.
diff --git a/packages/abstractions/tests/test_request_header.py b/packages/abstractions/tests/test_request_header.py
index dad8095d..2582f43e 100644
--- a/packages/abstractions/tests/test_request_header.py
+++ b/packages/abstractions/tests/test_request_header.py
@@ -98,8 +98,8 @@ def test_add_single_value_header_to_existing_header():
     headers = HeadersCollection()
     headers.add("content-type", "value1")
     headers.add("content-type", "value2")
-    assert {"value2"} <= headers.try_get("content-type")
-    assert {"value2"} <= headers.get("content-type")
+    assert headers.try_get("content-type") == {"value2"}
+    assert headers.get("content-type") == {"value2"}
     assert headers.contains("content-type")
     assert headers.count() == 1
     
@@ -180,4 +180,13 @@ def test_adds_headers_from_instance():
     assert headers.contains("header3")
     assert headers.contains("header4")
     assert headers.count() == 4
-    assert headers.keys() == ["header1", "header2", "header3", "header4"]
\ No newline at end of file
+    assert headers.keys() == ["header1", "header2", "header3", "header4"]
+def test_add_all_replaces_single_value_header():
+    """Tests that add_all keeps a single value header to one value
+    """
+    headers = HeadersCollection()
+    headers.add("Content-Type", "application/json")
+    other = HeadersCollection()
+    other.add("Content-Type", "text/plain")
+    headers.add_all(other)
+    assert headers.get("content-type") == {"text/plain"}