diff --git a/libs/langchain/langchain/retrievers/multi_query.py b/libs/langchain/langchain/retrievers/multi_query.py
index c98e6f9dbce..79ed26d1375 100644
--- a/libs/langchain/langchain/retrievers/multi_query.py
+++ b/libs/langchain/langchain/retrievers/multi_query.py
@@ -1,6 +1,7 @@
 import json
 import logging
-from typing import List
+from functools import partial
+from typing import Callable, List
 
 from langchain.callbacks.manager import CallbackManagerForRetrieverRun
 from langchain.chains.llm import LLMChain
@@ -137,7 +138,11 @@ class MultiQueryRetriever(BaseRetriever):
             documents.extend(docs)
         return documents
 
-    def unique_union(self, documents: List[Document]) -> List[Document]:
+    def unique_union(
+        self,
+        documents: List[Document],
+        metadata_encoder: Callable = partial(json.dumps, sort_keys=True),
+    ) -> List[Document]:
         """Get unique Documents.
 
         Args:
@@ -149,8 +154,7 @@ class MultiQueryRetriever(BaseRetriever):
         # Create a dictionary with page_content as keys to remove duplicates
         # TODO: Add Document ID property (e.g., UUID)
         unique_documents_dict = {
-            (doc.page_content, json.dumps(doc.metadata, sort_keys=True)): doc
-            for doc in documents
+            (doc.page_content, metadata_encoder(doc.metadata)): doc for doc in documents
         }
 
         unique_documents = list(unique_documents_dict.values())