Reject duplicate client IDs

internal/server/server.go

@@ -245,7 +245,15 @@ func (r *Room) HandleConn(ctx context.Context, playerID uuid.UUID, nickname stri
 
 	g, ctx := errgroup.WithContext(ctx)
 
+	setup := func() (success bool) {
 		r.mu.Lock()
+		defer r.mu.Unlock()
+
+		if _, ok := r.players[playerID]; ok {
+			ctxlog.Warn(ctx, "client with this ID already exists")
+			return false
+		}
+
 		r.players[playerID] = func(s protocol.ServerNote) {
 			if ctx.Err() != nil {
 				return
@@ -264,7 +272,12 @@ func (r *Room) HandleConn(ctx context.Context, playerID uuid.UUID, nickname stri
 		}
 		r.room.AddPlayer(playerID, nickname)
 		r.sendAll()
-	r.mu.Unlock()
+		return true
+	}
+
+	if !setup() {
+		return
+	}
 
 	defer func() {
 		r.mu.Lock()