diff --git a/roomserver/internal/input/input_events.go b/roomserver/internal/input/input_events.go
index 67031609f..6a5d9d264 100644
--- a/roomserver/internal/input/input_events.go
+++ b/roomserver/internal/input/input_events.go
@@ -17,6 +17,7 @@
 package input
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 
@@ -26,6 +27,7 @@ import (
 	"github.com/matrix-org/dendrite/roomserver/state"
 	"github.com/matrix-org/dendrite/roomserver/types"
 	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/util"
 	"github.com/sirupsen/logrus"
 )
 
@@ -44,6 +46,28 @@ func (r *Inputer) processRoomEvent(
 	headered := input.Event
 	event := headered.Unwrap()
 
+	// if we have already got this event then do not process it again, if the input kind is an outlier.
+	// Outliers contain no extra information which may warrant a re-processing.
+	if input.Kind == api.KindOutlier {
+		evs, err := r.DB.EventsFromIDs(ctx, []string{event.EventID()})
+		if err == nil && len(evs) == 1 {
+			// check hash matches if we're on early room versions where the event ID was a random string
+			idFormat, err := headered.RoomVersion.EventIDFormat()
+			if err == nil {
+				switch idFormat {
+				case gomatrixserverlib.EventIDFormatV1:
+					if bytes.Equal(event.EventReference().EventSHA256, evs[0].EventReference().EventSHA256) {
+						util.GetLogger(ctx).WithField("event_id", event.EventID()).Infof("Already processed event; ignoring")
+						return event.EventID(), nil
+					}
+				default:
+					util.GetLogger(ctx).WithField("event_id", event.EventID()).Infof("Already processed event; ignoring")
+					return event.EventID(), nil
+				}
+			}
+		}
+	}
+
 	// Check that the event passes authentication checks and work out
 	// the numeric IDs for the auth events.
 	isRejected := false
diff --git a/roomserver/internal/input/input_latest_events.go b/roomserver/internal/input/input_latest_events.go
index 5adcd0877..f76b0a0b4 100644
--- a/roomserver/internal/input/input_latest_events.go
+++ b/roomserver/internal/input/input_latest_events.go
@@ -233,7 +233,7 @@ func (u *latestEventsUpdater) latestState() error {
 	if err != nil {
 		return fmt.Errorf("roomState.DifferenceBetweenStateSnapshots: %w", err)
 	}
-	if len(u.removed) > len(u.added) {
+	if !u.stateAtEvent.Overwrite && len(u.removed) > len(u.added) {
 		// This really shouldn't happen.
 		// TODO: What is ultimately the best way to handle this situation?
 		logrus.Errorf(
diff --git a/roomserver/storage/shared/latest_events_updater.go b/roomserver/storage/shared/latest_events_updater.go
index b316f639d..8825dc464 100644
--- a/roomserver/storage/shared/latest_events_updater.go
+++ b/roomserver/storage/shared/latest_events_updater.go
@@ -70,16 +70,14 @@ func (u *LatestEventsUpdater) CurrentStateSnapshotNID() types.StateSnapshotNID {
 	return u.currentStateSnapshotNID
 }
 
-// StorePreviousEvents implements types.RoomRecentEventsUpdater
+// StorePreviousEvents implements types.RoomRecentEventsUpdater - This must be called from a Writer
 func (u *LatestEventsUpdater) StorePreviousEvents(eventNID types.EventNID, previousEventReferences []gomatrixserverlib.EventReference) error {
-	return u.d.Writer.Do(u.d.DB, u.txn, func(txn *sql.Tx) error {
-		for _, ref := range previousEventReferences {
-			if err := u.d.PrevEventsTable.InsertPreviousEvent(u.ctx, txn, ref.EventID, ref.EventSHA256, eventNID); err != nil {
-				return fmt.Errorf("u.d.PrevEventsTable.InsertPreviousEvent: %w", err)
-			}
+	for _, ref := range previousEventReferences {
+		if err := u.d.PrevEventsTable.InsertPreviousEvent(u.ctx, u.txn, ref.EventID, ref.EventSHA256, eventNID); err != nil {
+			return fmt.Errorf("u.d.PrevEventsTable.InsertPreviousEvent: %w", err)
 		}
-		return nil
-	})
+	}
+	return nil
 }
 
 // IsReferenced implements types.RoomRecentEventsUpdater
diff --git a/roomserver/storage/shared/storage.go b/roomserver/storage/shared/storage.go
index f2be8b3cf..51dcb8887 100644
--- a/roomserver/storage/shared/storage.go
+++ b/roomserver/storage/shared/storage.go
@@ -492,15 +492,32 @@ func (d *Database) StoreEvent(
 		if roomInfo == nil && len(prevEvents) > 0 {
 			return 0, types.StateAtEvent{}, nil, "", fmt.Errorf("expected room %q to exist", event.RoomID())
 		}
+		// Create an updater - NB: on sqlite this WILL create a txn as we are directly calling the shared DB form of
+		// GetLatestEventsForUpdate - not via the SQLiteDatabase form which has `nil` txns. This
+		// function only does SELECTs though so the created txn (at this point) is just a read txn like
+		// any other so this is fine. If we ever update GetLatestEventsForUpdate or NewLatestEventsUpdater
+		// to do writes however then this will need to go inside `Writer.Do`.
 		updater, err = d.GetLatestEventsForUpdate(ctx, *roomInfo)
 		if err != nil {
 			return 0, types.StateAtEvent{}, nil, "", fmt.Errorf("NewLatestEventsUpdater: %w", err)
 		}
-		if err = updater.StorePreviousEvents(eventNID, prevEvents); err != nil {
-			return 0, types.StateAtEvent{}, nil, "", fmt.Errorf("updater.StorePreviousEvents: %w", err)
+		// Ensure that we atomically store prev events AND commit them. If we don't wrap StorePreviousEvents
+		// and EndTransaction in a writer then it's possible for a new write txn to be made between the two
+		// function calls which will then fail with 'database is locked'. This new write txn would HAVE to be
+		// something like SetRoomAlias/RemoveRoomAlias as normal input events are already done sequentially due to
+		// SupportsConcurrentRoomInputs() == false on sqlite, though this does not apply to setting room aliases
+		// as they don't go via InputRoomEvents
+		err = d.Writer.Do(d.DB, updater.txn, func(txn *sql.Tx) error {
+			if err = updater.StorePreviousEvents(eventNID, prevEvents); err != nil {
+				return fmt.Errorf("updater.StorePreviousEvents: %w", err)
+			}
+			succeeded := true
+			err = sqlutil.EndTransaction(updater, &succeeded)
+			return err
+		})
+		if err != nil {
+			return 0, types.StateAtEvent{}, nil, "", err
 		}
-		succeeded := true
-		err = sqlutil.EndTransaction(updater, &succeeded)
 	}
 
 	return roomNID, types.StateAtEvent{