diff --git a/changelog.d/19464.bugfix b/changelog.d/19464.bugfix
new file mode 100644
index 00000000000..147e0b6ec79
--- /dev/null
+++ b/changelog.d/19464.bugfix
@@ -0,0 +1 @@
+Fix `/sync` `timeline` state duplicated in `state` when lazy-loading.
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index a32748c2a9c..a56ab4be3b7 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -1046,6 +1046,13 @@ async def compute_state_delta(
                     if event.sender not in first_event_by_sender_map:
                         first_event_by_sender_map[event.sender] = event
 
+                    # Add to the `timeline_state` first before we check whether members
+                    # are in the `timeline` just below. Otherwise, we will end up with
+                    # extra things in `state` that should have only been in the
+                    # `timeline`.
+                    if event.is_state():
+                        timeline_state[(event.type, event.state_key)] = event.event_id
+
                     # When using `state_after`, there is no special treatment with
                     # regards to state also being in the `timeline`. Always fetch
                     # relevant membership regardless of whether the state event is in
@@ -1061,9 +1068,6 @@ async def compute_state_delta(
                         members_to_fetch.add(event.sender)
                     # FIXME: we also care about invite targets etc.
 
-                    if event.is_state():
-                        timeline_state[(event.type, event.state_key)] = event.event_id
-
             else:
                 timeline_state = {
                     (event.type, event.state_key): event.event_id