diff --git a/Chat/Client/Singleton.cs b/Chat/Client/Singleton.cs
index 98a6e22..0dbb40f 100644
--- a/Chat/Client/Singleton.cs
+++ b/Chat/Client/Singleton.cs
@@ -109,6 +109,12 @@ namespace Client
 						ChatPacket packet = new ChatPacket(dataBuffer);
 						ChatResponsed?.Invoke(packet, EventArgs.Empty);
 					}
+					else if (packetType == PacketType.Duplicate)
+					{
+						socket.Shutdown(SocketShutdown.Send);
+						MessageBox.Show("중복접속");
+						Environment.Exit(0);
+					}
 				}
 			}
 			catch (Exception ex)
diff --git a/Chat/Core/DuplicatePacket.cs b/Chat/Core/DuplicatePacket.cs
new file mode 100644
index 0000000..7829a43
--- /dev/null
+++ b/Chat/Core/DuplicatePacket.cs
@@ -0,0 +1,30 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Net;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Core
+{
+	public class DuplicatePacket : IPacket
+	{
+		public byte[] Serialize()
+		{
+			byte[] packetType = BitConverter.GetBytes(IPAddress.HostToNetworkOrder((short)PacketType.Duplicate));
+
+			short dataSize = (short)(packetType.Length);
+			byte[] header = BitConverter.GetBytes(IPAddress.HostToNetworkOrder(dataSize));
+
+			byte[] buffer = new byte[2 + dataSize];
+
+			int cursor = 0;
+			Array.Copy(header, 0, buffer, cursor, header.Length);
+			cursor += header.Length;
+
+			Array.Copy(packetType, 0, buffer, cursor, packetType.Length);
+
+			return buffer;
+		}
+	}
+}
diff --git a/Chat/Core/PServer.cs b/Chat/Core/PServer.cs
index b617926..43e3c5a 100644
--- a/Chat/Core/PServer.cs
+++ b/Chat/Core/PServer.cs
@@ -85,13 +85,18 @@ namespace Core
 					{
 						LoginRequestPacket requestPacket = new LoginRequestPacket(dataBuffer);
 
-						bool result = Clients.TryAdd(requestPacket.Id, clientSocket);
+						Clients.AddOrUpdate(requestPacket.Id, clientSocket, (oldKey, oldValue) =>
+						{
+							DuplicatePacket packet = new DuplicatePacket();
+							oldValue.Send(packet.Serialize());
+							return clientSocket;
+						});
 						Console.WriteLine($"[{DateTime.Now}] LoginRequest - Id: {requestPacket.Id}, Nickname: {requestPacket.NickName}");
 
 						id = requestPacket.Id;
 						nickname = requestPacket.NickName;
 
-						LoginResponsePacket responsePacket = new LoginResponsePacket(result ? StatusCode.Success : StatusCode.Failed);
+						LoginResponsePacket responsePacket = new LoginResponsePacket(StatusCode.Success);
 						await clientSocket.SendAsync(responsePacket.Serialize(), SocketFlags.None);
 					}
 					else if (packetType == PacketType.CreateRoomRequest)
@@ -201,7 +206,9 @@ namespace Core
 
 		private async Task Remove(string id, string nickname, string roomName, Socket clientSocket)
 		{
-			Clients.TryRemove(id, out _);
+			if (Clients.TryGetValue(id, out var client) && client == clientSocket)
+				Clients.TryRemove(id, out _);
+
 			if (Rooms.TryGetValue(roomName, out var room))
 			{
 				UserLeavePacket packet = new UserLeavePacket(nickname);
diff --git a/Chat/Core/PacketType.cs b/Chat/Core/PacketType.cs
index d1be89e..a686746 100644
--- a/Chat/Core/PacketType.cs
+++ b/Chat/Core/PacketType.cs
@@ -18,6 +18,7 @@ namespace Core
 		EnterRoomResponse,
 		UserEnter,
 		UserLeave,
-		Chat
+		Chat,
+		Duplicate
 	}
 }