diff --git a/src/tty/ntapi_tty_join_session.c b/src/tty/ntapi_tty_join_session.c
index 77bcc3c..2bcefc4 100644
--- a/src/tty/ntapi_tty_join_session.c
+++ b/src/tty/ntapi_tty_join_session.c
@@ -17,10 +17,14 @@ int32_t __stdcall __ntapi_tty_join_session(
 	nt_status			status;
 	ntapi_internals *		__internals;
 
-	/* init */
+	/* init, primary session semantics */
 	__internals = __ntapi_internals();
 
 	if (type == NT_TTY_SESSION_PRIMARY) {
+		if (__internals->rtdata)
+			if (__internals->rtdata->hsession)
+				return NT_STATUS_PORT_ALREADY_SET;
+
 		hport = hport ? hport : &__internals->hport_tty_session;
 		port_name = port_name ? port_name : __internals->subsystem;
 	}