diff --git a/apps/client/src/hooks/useAppFlow.ts b/apps/client/src/hooks/useAppFlow.ts
index db815c9..2bcc332 100644
--- a/apps/client/src/hooks/useAppFlow.ts
+++ b/apps/client/src/hooks/useAppFlow.ts
@@ -57,6 +57,11 @@
       setIsJoining(false);
       if (payload.reason === "full") {
         setJoinErrorMessage(`ルーム ${payload.roomId} は満員です`);
+        return;
+      }
+
+      if (payload.reason === "duplicate") {
+        setJoinErrorMessage(`ルーム ${payload.roomId} への参加要求が重複しました`);
       }
     };
     const handleGameStart = () => {
diff --git a/apps/server/src/network/handlers/room/registerRoomHandlers.ts b/apps/server/src/network/handlers/room/registerRoomHandlers.ts
index 33dd81c..7d7fbbf 100644
--- a/apps/server/src/network/handlers/room/registerRoomHandlers.ts
+++ b/apps/server/src/network/handlers/room/registerRoomHandlers.ts
@@ -43,7 +43,7 @@
       publishRoomUpdate: roomPublisher.publishRoomUpdate,
     });
 
-    // 満員拒否時はソケットのルーム参加状態を巻き戻す
+    // 参加拒否時は理由を通知する
     if (joinResult.status === "full") {
       socket.leave(roomId);
       roomPublisher.publishJoinRejected({
@@ -56,6 +56,20 @@
         roomId,
         socketId: socket.id,
       });
+      return;
+    }
+
+    if (joinResult.status === "duplicate") {
+      roomPublisher.publishJoinRejected({
+        roomId,
+        reason: "duplicate",
+      });
+      logEvent("Network", {
+        event: "JOIN_ROOM",
+        result: "rejected_duplicate",
+        roomId,
+        socketId: socket.id,
+      });
     }
   });
 };
diff --git a/packages/shared/src/domains/room/room.type.ts b/packages/shared/src/domains/room/room.type.ts
index 6226444..f69dcbd 100644
--- a/packages/shared/src/domains/room/room.type.ts
+++ b/packages/shared/src/domains/room/room.type.ts
@@ -25,7 +25,7 @@
 }
 
 // ルーム参加拒否理由型
-export type JoinRoomRejectedReason = "full";
+export type JoinRoomRejectedReason = "full" | "duplicate";
 
 // ルーム参加拒否通知ペイロード型
 export interface JoinRoomRejectedPayload {