diff --git a/cockatrice/src/window_main.cpp b/cockatrice/src/window_main.cpp
index 6d090e8f..3d39a785 100644
--- a/cockatrice/src/window_main.cpp
+++ b/cockatrice/src/window_main.cpp
@@ -176,6 +176,7 @@ void MainWindow::serverError(ResponseCode r)
 {
 	switch (r) {
 		case RespWrongPassword: QMessageBox::critical(this, tr("Error"), tr("Invalid login data.")); break;
+		case RespWouldOverwriteOldSession: QMessageBox::critical(this, tr("Error"), tr("There is already an active session using this user name.\nPlease close that session first and re-login.")); break;
 		default: ;
 	}
 }
diff --git a/common/protocol.cpp b/common/protocol.cpp
index 1185e90b..726803e1 100644
--- a/common/protocol.cpp
+++ b/common/protocol.cpp
@@ -267,6 +267,7 @@ void ProtocolResponse::initializeHash()
 	responseHash.insert("only_buddies", RespOnlyBuddies);
 	responseHash.insert("user_level_too_low", RespUserLevelTooLow);
 	responseHash.insert("in_ignore_list", RespInIgnoreList);
+	responseHash.insert("would_overwrite_old_session", RespWouldOverwriteOldSession);
 }
 
 Response_JoinRoom::Response_JoinRoom(int _cmdId, ResponseCode _responseCode, ServerInfo_Room *_roomInfo)
diff --git a/common/protocol_datastructures.h b/common/protocol_datastructures.h
index 9fcfb295..dea6988c 100644
--- a/common/protocol_datastructures.h
+++ b/common/protocol_datastructures.h
@@ -8,7 +8,7 @@
 
 class DeckList;
 
-enum ResponseCode { RespNothing, RespOk, RespInternalError, RespInvalidCommand, RespInvalidData, RespNameNotFound, RespLoginNeeded, RespFunctionNotAllowed, RespGameNotStarted, RespGameFull, RespContextError, RespWrongPassword, RespSpectatorsNotAllowed, RespOnlyBuddies, RespUserLevelTooLow, RespInIgnoreList };
+enum ResponseCode { RespNothing, RespOk, RespInternalError, RespInvalidCommand, RespInvalidData, RespNameNotFound, RespLoginNeeded, RespFunctionNotAllowed, RespGameNotStarted, RespGameFull, RespContextError, RespWrongPassword, RespSpectatorsNotAllowed, RespOnlyBuddies, RespUserLevelTooLow, RespInIgnoreList, RespWouldOverwriteOldSession };
 
 // PrivateZone: Contents of the zone are always visible to the owner,
 // but not to anyone else.
diff --git a/common/server.cpp b/common/server.cpp
index ac4b5e63..6e6bf488 100644
--- a/common/server.cpp
+++ b/common/server.cpp
@@ -46,8 +46,11 @@ AuthenticationResult Server::loginUser(Server_ProtocolHandler *session, QString
 	
 	if (authState == PasswordRight) {
 		Server_ProtocolHandler *oldSession = users.value(name);
-		if (oldSession)
+		if (oldSession) {
+			if (!(oldSession->getUserInfo()->getUserLevel() & ServerInfo_User::IsRegistered))
+				return WouldOverwriteOldSession;
 			delete oldSession; // ~Server_ProtocolHandler() will call Server::removeClient
+		}
 	} else if (authState == UnknownUser) {
 		// Change user name so that no two users have the same names,
 		// don't interfere with registered user names though.
diff --git a/common/server.h b/common/server.h
index 2f4527b9..3816cef4 100644
--- a/common/server.h
+++ b/common/server.h
@@ -10,7 +10,7 @@ class Server_Room;
 class Server_ProtocolHandler;
 class ServerInfo_User;
 
-enum AuthenticationResult { PasswordWrong = 0, PasswordRight = 1, UnknownUser = 2 };
+enum AuthenticationResult { PasswordWrong = 0, PasswordRight = 1, UnknownUser = 2, WouldOverwriteOldSession = 3 };
 
 class Server : public QObject
 {
diff --git a/common/server_protocolhandler.cpp b/common/server_protocolhandler.cpp
index 71d56a05..7f0ebe4a 100644
--- a/common/server_protocolhandler.cpp
+++ b/common/server_protocolhandler.cpp
@@ -237,6 +237,8 @@ ResponseCode Server_ProtocolHandler::cmdLogin(Command_Login *cmd, CommandContain
 	authState = server->loginUser(this, userName, cmd->getPassword());
 	if (authState == PasswordWrong)
 		return RespWrongPassword;
+	if (authState == WouldOverwriteOldSession)
+		return RespWouldOverwriteOldSession;
 
 	enqueueProtocolItem(new Event_ServerMessage(server->getLoginMessage()));