diff --git a/fastapi/dependencies/utils.py b/fastapi/dependencies/utils.py
index d205d17fa..43e74452b 100644
--- a/fastapi/dependencies/utils.py
+++ b/fastapi/dependencies/utils.py
@@ -837,6 +837,7 @@ async def _extract_form_body(
     values = {}
     first_field = body_fields[0]
     first_field_info = first_field.field_info
+    processed_keys = set()
 
     for field in body_fields:
         value = _get_multidict_value(field, received_body)
@@ -865,10 +866,11 @@ async def _extract_form_body(
                 for sub_value in value:
                     tg.start_soon(process_fn, sub_value.read)
             value = serialize_sequence_value(field=field, value=results)
+        processed_keys.add(field.alias)
         if value is not None:
             values[field.alias] = value
     for key, value in received_body.items():
-        if key not in values:
+        if key not in processed_keys:
             values[key] = value
     return values