Fix race condition in registration leading to duplicate create_auth calls

src/network/serverpackethandler.cpp

@@ -1495,8 +1495,19 @@ void Server::handleCommand_FirstSrp(NetworkPacket* pkt)
 		}
 
 		std::string initial_ver_key;
-
 		initial_ver_key = encode_srp_verifier(verification_key, salt);
+
+		// It is possible for multiple connections to get this far with the same
+		// player name. In the end only one player with a given name will be emerged
+		// (see Server::StateTwoClientInit) but we still have to be careful here.
+		if (m_script->getAuth(playername, nullptr, nullptr)) {
+			// Another client beat us to it
+			actionstream << "Server: Client from " << addr_s
+				<< " tried to register " << playername << " a second time."
+				<< std::endl;
+			DenyAccess(peer_id, SERVER_ACCESSDENIED_ALREADY_CONNECTED);
+			return;
+		}
 		m_script->createAuth(playername, initial_ver_key);
 		m_script->on_authplayer(playername, addr_s, true);