fix: Add id field to sort config for consisteny pagination

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## [Unreleased]
 
+- Added `id` field as secondary sort to sort config to ensure unique pagination tokens. [#421](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/421)
 - Added default environment variable `STAC_ITEM_LIMIT` to SFEOS for result limiting of returned items and STAC collections [#419](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/419)
 
 ## [v6.2.0] - 2025-08-27

stac_fastapi/sfeos_helpers/stac_fastapi/sfeos_helpers/database/query.py

@@ -80,11 +80,14 @@ def populate_sort_shared(sortby: List) -> Optional[Dict[str, Dict[str, str]]]:
         This function transforms a list of sort specifications into the format required by
         Elasticsearch/OpenSearch for sorting query results. The returned dictionary can be
         directly used in search requests.
+        Always includes 'id' as secondary sort to ensure unique pagination tokens.
     """
     if sortby:
-        return {s.field: {"order": s.direction} for s in sortby}
+        sort_config = {s.field: {"order": s.direction} for s in sortby}
+        sort_config.setdefault("id", {"order": "asc"})
+        return sort_config
     else:
-        return None
+        return {"id": {"order": "asc"}}
 
 
 def add_collections_to_body(