diff --git a/clientapi/routing/routing.go b/clientapi/routing/routing.go
index 27174dd98..4262b18ac 100644
--- a/clientapi/routing/routing.go
+++ b/clientapi/routing/routing.go
@@ -20,8 +20,6 @@ import (
 	"strings"
 
 	"github.com/gorilla/mux"
-	"github.com/matrix-org/dendrite/setup/base"
-	userapi "github.com/matrix-org/dendrite/userapi/api"
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/gomatrixserverlib/fclient"
 	"github.com/matrix-org/util"
@@ -30,6 +28,9 @@ import (
 	"github.com/sirupsen/logrus"
 	"golang.org/x/sync/singleflight"
 
+	"github.com/matrix-org/dendrite/setup/base"
+	userapi "github.com/matrix-org/dendrite/userapi/api"
+
 	appserviceAPI "github.com/matrix-org/dendrite/appservice/api"
 	"github.com/matrix-org/dendrite/clientapi/api"
 	"github.com/matrix-org/dendrite/clientapi/auth"
@@ -86,6 +87,12 @@ func Setup(
 		unstableFeatures["org.matrix."+msc] = true
 	}
 
+	// singleflight protects /join endpoints from being invoked
+	// multiple times from the same user and room, otherwise
+	// a state reset can occur. This also avoids unneeded
+	// state calculations.
+	// TODO: actually fix this in the roomserver, as there are
+	// 		 possibly other ways that can result in a stat reset.
 	sf := singleflight.Group{}
 
 	if cfg.Matrix.WellKnownClientName != "" {
@@ -268,11 +275,14 @@ func Setup(
 			if err != nil {
 				return util.ErrorResponse(err)
 			}
+			// Only execute a join for roomIDOrAlias and UserID once. If there is a join in progress
+			// it waits for it to complete and returns that result for subsequent requests.
 			resp, _, _ := sf.Do(vars["roomIDOrAlias"]+device.UserID, func() (any, error) {
 				return JoinRoomByIDOrAlias(
 					req, device, rsAPI, userAPI, vars["roomIDOrAlias"],
 				), nil
 			})
+			// drop the result from the cache, so subsequent requests go through as normal.
 			sf.Forget(vars["roomIDOrAlias"] + device.UserID)
 			return resp.(util.JSONResponse)
 		}, httputil.WithAllowGuests()),
@@ -308,11 +318,14 @@ func Setup(
 			if err != nil {
 				return util.ErrorResponse(err)
 			}
+			// Only execute a join for roomID and UserID once. If there is a join in progress
+			// it waits for it to complete and returns that result for subsequent requests.
 			resp, _, _ := sf.Do(vars["roomID"]+device.UserID, func() (any, error) {
 				return JoinRoomByIDOrAlias(
 					req, device, rsAPI, userAPI, vars["roomID"],
 				), nil
 			})
+			// drop the result from the cache, so subsequent requests go through as normal.
 			sf.Forget(vars["roomID"] + device.UserID)
 			return resp.(util.JSONResponse)
 		}, httputil.WithAllowGuests()),
diff --git a/go.mod b/go.mod
index 0fac0b999..c3a3f846b 100644
--- a/go.mod
+++ b/go.mod
@@ -45,6 +45,7 @@ require (
 	golang.org/x/crypto v0.8.0
 	golang.org/x/image v0.5.0
 	golang.org/x/mobile v0.0.0-20221020085226-b36e6246172e
+	golang.org/x/sync v0.1.0
 	golang.org/x/term v0.7.0
 	gopkg.in/h2non/bimg.v1 v1.1.9
 	gopkg.in/yaml.v2 v2.4.0
@@ -124,7 +125,6 @@ require (
 	golang.org/x/mod v0.8.0 // indirect
 	golang.org/x/net v0.9.0 // indirect
 	golang.org/x/sys v0.7.0 // indirect
-	golang.org/x/sync v0.1.0 // indirect
 	golang.org/x/text v0.9.0 // indirect
 	golang.org/x/time v0.3.0 // indirect
 	golang.org/x/tools v0.6.0 // indirect
diff --git a/go.sum b/go.sum
index ad3017f2f..e823cf271 100644
--- a/go.sum
+++ b/go.sum
@@ -605,6 +605,7 @@ golang.org/x/sync v0.0.0-20201207232520-09787c993a3a/go.mod h1:RxMgew5VJxzue5/jJ
 golang.org/x/sync v0.0.0-20210220032951-036812b2e83c/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sync v0.0.0-20220722155255-886fb9371eb4/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sync v0.1.0 h1:wsuoTGHzEhffawBOhz5CYhcrV4IdKZbEyZjBMuTp12o=
+golang.org/x/sync v0.1.0/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sys v0.0.0-20180830151530-49385e6e1522/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20180905080454-ebe1bf3edb33/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20181116152217-5ac8a444bdc5/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=