diff --git a/synapse/storage/databases/state/store.py b/synapse/storage/databases/state/store.py
index e1868552e5..6bc384c02c 100644
--- a/synapse/storage/databases/state/store.py
+++ b/synapse/storage/databases/state/store.py
@@ -247,6 +247,46 @@ class StateGroupDataStore(StateBackgroundUpdateStore, SQLBaseStore):
 
         return state_filter.filter_state(state_dict_ids), not missing_types
 
+    def _get_state_for_group_gather_inflight_requests(
+        self, group: int, state_filter: StateFilter
+    ) -> Tuple[Sequence[ObservableDeferred[StateMap[str]]], StateFilter]:
+        """
+        Attempts to gather in-flight requests and re-use them to retrieve state
+        for the given state group, filtered with the given state filter.
+
+        Returns:
+            tuple (
+                sequence of ObservableDeferreds to observe,
+                StateFilter representing what is left over (what else needs
+                to be requested to fulfil the request)
+            )
+        """
+
+        inflight_requests = self._state_group_inflight_requests.get(group)
+        if inflight_requests is None:
+            # no requests for this group, need to retrieve it all ourselves
+            return (), state_filter
+
+        state_filter_left_over = state_filter
+        reusable_requests = []
+        for (
+            request_state_filter,
+            request_deferred,
+        ) in inflight_requests.items():
+            new_state_filter_left_over = state_filter_left_over.approx_difference(
+                request_state_filter
+            )
+            if new_state_filter_left_over != state_filter_left_over:
+                # reusing this request narrows our StateFilter down a bit.
+                reusable_requests.append(request_deferred)
+            state_filter_left_over = new_state_filter_left_over
+            if state_filter_left_over == StateFilter.none():
+                # we have managed to collect enough of the in-flight requests
+                # to cover our StateFilter and give us the state we need.
+                break
+
+        return reusable_requests, state_filter_left_over
+
     async def _get_state_for_group_fire_request(
         self, group: int, state_filter: StateFilter
     ) -> StateMap[str]: