diff --git a/server/src/client.ts b/server/src/client.ts
index 930518e..7e94303 100644
--- a/server/src/client.ts
+++ b/server/src/client.ts
@@ -114,7 +114,7 @@ export class Client extends EventEmitter {
 		switch (msg.op) {
 			case MSAgentProtocolMessageType.Join: {
 				let joinMsg = msg as MSAgentJoinMessage;
-				if (!joinMsg.data || !joinMsg.data.username || !joinMsg.data.username) {
+				if (this.username !== null || !joinMsg.data || !joinMsg.data.username || !joinMsg.data.username) {
 					this.socket.close();
 					return;
 				}