fix: duplicated header completions if languages contains same words

Author: d-biehl

packages/language_server/src/robotcode/language_server/robotframework/parts/completion.py

@@ -411,7 +411,7 @@ async def create_headers_completion_items(self, range: Optional[Range]) -> List[
             ):
                 languages = [v for v in languages if v.code != "en"]
 
-            headers = itertools.chain(*(lang.headers.keys() for lang in languages))
+            headers = set(itertools.chain(*(lang.headers.keys() for lang in languages)))
 
         return [
             CompletionItem(