diff --git a/api_signaling.go b/api_signaling.go
index d9ca99a..1c48c78 100644
--- a/api_signaling.go
+++ b/api_signaling.go
@@ -24,6 +24,7 @@ package signaling
 import (
 	"encoding/json"
 	"fmt"
+	"log"
 	"net/url"
 	"sort"
 	"strings"
@@ -220,9 +221,9 @@ func (r *ServerMessage) String() string {
 }
 
 type Error struct {
-	Code    string      `json:"code"`
-	Message string      `json:"message"`
-	Details interface{} `json:"details,omitempty"`
+	Code    string          `json:"code"`
+	Message string          `json:"message"`
+	Details json.RawMessage `json:"details,omitempty"`
 }
 
 func NewError(code string, message string) *Error {
@@ -230,10 +231,19 @@ func NewError(code string, message string) *Error {
 }
 
 func NewErrorDetail(code string, message string, details interface{}) *Error {
+	var rawDetails json.RawMessage
+	if details != nil {
+		var err error
+		if rawDetails, err = json.Marshal(details); err != nil {
+			log.Printf("Could not marshal details %+v for error %s with %s: %s", details, code, message, err)
+			return NewError("internal_error", "Could not marshal error details")
+		}
+	}
+
 	return &Error{
 		Code:    code,
 		Message: message,
-		Details: details,
+		Details: rawDetails,
 	}
 }
 
@@ -511,6 +521,10 @@ type RoomServerMessage struct {
 	Properties *json.RawMessage `json:"properties,omitempty"`
 }
 
+type RoomErrorDetails struct {
+	Room *RoomServerMessage `json:"room"`
+}
+
 // Type "message"
 
 const (
diff --git a/clientsession.go b/clientsession.go
index 9bb816e..e0243b0 100644
--- a/clientsession.go
+++ b/clientsession.go
@@ -417,6 +417,36 @@ func (s *ClientSession) SubscribeEvents() error {
 	return s.events.RegisterSessionListener(s.publicId, s.backend, s)
 }
 
+func (s *ClientSession) UpdateRoomSessionId(roomSessionId string) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	if s.roomSessionId == roomSessionId {
+		return nil
+	}
+
+	if err := s.hub.roomSessions.SetRoomSession(s, roomSessionId); err != nil {
+		return err
+	}
+
+	if roomSessionId != "" {
+		if room := s.GetRoom(); room != nil {
+			log.Printf("Session %s updated room session id to %s in room %s", s.PublicId(), roomSessionId, room.Id())
+		} else {
+			log.Printf("Session %s updated room session id to %s in unknown room", s.PublicId(), roomSessionId)
+		}
+	} else {
+		if room := s.GetRoom(); room != nil {
+			log.Printf("Session %s cleared room session id in room %s", s.PublicId(), room.Id())
+		} else {
+			log.Printf("Session %s cleared room session id in unknown room", s.PublicId())
+		}
+	}
+
+	s.roomSessionId = roomSessionId
+	return nil
+}
+
 func (s *ClientSession) SubscribeRoomEvents(roomid string, roomSessionId string) error {
 	s.mu.Lock()
 	defer s.mu.Unlock()
diff --git a/docs/standalone-signaling-api-v1.md b/docs/standalone-signaling-api-v1.md
index 69ea27e..a53dd26 100644
--- a/docs/standalone-signaling-api-v1.md
+++ b/docs/standalone-signaling-api-v1.md
@@ -460,6 +460,26 @@ Message format (Server -> Client):
   the current room or the properties of a room change.
 
 
+Message format (Server -> Client if already joined before):
+
+    {
+      "id": "unique-request-id-from-request",
+      "type": "error",
+      "error": {
+        "code": "already_joined",
+        "message": "Human readable error message",
+        "details": {
+          "roomid": "the-room-id",
+          "properties": {
+            ...additional room properties...
+          }
+        }
+      }
+    }
+
+- Sent if a client tried to join a room it is already in.
+
+
 ### Backend validation
 
 Rooms are managed by the Nextcloud backend, so the signaling server has to
diff --git a/hub.go b/hub.go
index 7a4e51a..ee6b1eb 100644
--- a/hub.go
+++ b/hub.go
@@ -1256,6 +1256,24 @@ func (h *Hub) processRoom(client *Client, message *ClientMessage) {
 	if session != nil {
 		if room := h.getRoomForBackend(roomId, session.Backend()); room != nil && room.HasSession(session) {
 			// Session already is in that room, no action needed.
+			roomSessionId := message.Room.SessionId
+			if roomSessionId == "" {
+				// TODO(jojo): Better make the session id required in the request.
+				log.Printf("User did not send a room session id, assuming session %s", session.PublicId())
+				roomSessionId = session.PublicId()
+			}
+
+			if err := session.UpdateRoomSessionId(roomSessionId); err != nil {
+				log.Printf("Error updating room session id for session %s: %s", session.PublicId(), err)
+			}
+			session.SendMessage(message.NewErrorServerMessage(
+				NewErrorDetail("already_joined", "Already joined this room.", &RoomErrorDetails{
+					Room: &RoomServerMessage{
+						RoomId:     room.id,
+						Properties: room.properties,
+					},
+				}),
+			))
 			return
 		}
 	}
diff --git a/hub_test.go b/hub_test.go
index 6f05d30..9bfa242 100644
--- a/hub_test.go
+++ b/hub_test.go
@@ -22,6 +22,7 @@
 package signaling
 
 import (
+	"bytes"
 	"context"
 	"crypto/ecdsa"
 	"crypto/ed25519"
@@ -58,6 +59,10 @@ const (
 	testTimeout = 10 * time.Second
 )
 
+var (
+	testRoomProperties = []byte("{\"prop1\":\"value1\"}")
+)
+
 var (
 	clusteredTests = []string{
 		"local",
@@ -403,8 +408,9 @@ func processRoomRequest(t *testing.T, w http.ResponseWriter, r *http.Request, re
 	response := &BackendClientResponse{
 		Type: "room",
 		Room: &BackendClientRoomResponse{
-			Version: BackendVersion,
-			RoomId:  request.Room.RoomId,
+			Version:    BackendVersion,
+			RoomId:     request.Room.RoomId,
+			Properties: (*json.RawMessage)(&testRoomProperties),
 		},
 	}
 	switch request.Room.RoomId {
@@ -2628,6 +2634,83 @@ func TestJoinRoom(t *testing.T) {
 	}
 }
 
+func TestJoinRoomTwice(t *testing.T) {
+	hub, _, _, server := CreateHubForTest(t)
+
+	client := NewTestClient(t, server, hub)
+	defer client.CloseWithBye()
+
+	if err := client.SendHello(testDefaultUserId); err != nil {
+		t.Fatal(err)
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), testTimeout)
+	defer cancel()
+
+	hello, err := client.RunUntilHello(ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Join room by id.
+	roomId := "test-room"
+	if room, err := client.JoinRoom(ctx, roomId); err != nil {
+		t.Fatal(err)
+	} else if room.Room.RoomId != roomId {
+		t.Fatalf("Expected room %s, got %s", roomId, room.Room.RoomId)
+	} else if !bytes.Equal(testRoomProperties, *room.Room.Properties) {
+		t.Fatalf("Expected room properties %s, got %s", string(testRoomProperties), string(*room.Room.Properties))
+	}
+
+	// We will receive a "joined" event.
+	if err := client.RunUntilJoined(ctx, hello.Hello); err != nil {
+		t.Error(err)
+	}
+
+	msg := &ClientMessage{
+		Id:   "ABCD",
+		Type: "room",
+		Room: &RoomClientMessage{
+			RoomId:    roomId,
+			SessionId: roomId + "-" + client.publicId + "-2",
+		},
+	}
+	if err := client.WriteJSON(msg); err != nil {
+		t.Fatal(err)
+	}
+
+	message, err := client.RunUntilMessage(ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := checkUnexpectedClose(err); err != nil {
+		t.Fatal(err)
+	}
+
+	if msg.Id != message.Id {
+		t.Errorf("expected message id %s, got %s", msg.Id, message.Id)
+	} else if err := checkMessageType(message, "error"); err != nil {
+		t.Fatal(err)
+	} else if expected := "already_joined"; message.Error.Code != expected {
+		t.Errorf("expected error %s, got %s", expected, message.Error.Code)
+	} else if message.Error.Details == nil {
+		t.Fatal("expected error details")
+	}
+
+	var roomMsg RoomErrorDetails
+	if err := json.Unmarshal(message.Error.Details, &roomMsg); err != nil {
+		t.Fatal(err)
+	} else if roomMsg.Room == nil {
+		t.Fatalf("expected room details, got %+v", message)
+	}
+
+	if roomMsg.Room.RoomId != roomId {
+		t.Fatalf("Expected room %s, got %+v", roomId, roomMsg.Room)
+	} else if !bytes.Equal(testRoomProperties, *roomMsg.Room.Properties) {
+		t.Fatalf("Expected room properties %s, got %s", string(testRoomProperties), string(*roomMsg.Room.Properties))
+	}
+}
+
 func TestExpectAnonymousJoinRoom(t *testing.T) {
 	hub, _, _, server := CreateHubForTest(t)