--- sys/netinet/in_mcast.c.orig	2009-08-03 08:13:06.000000000 +0000
+++ sys/netinet/in_mcast.c	2009-09-09 01:35:06.000000000 +0000
@@ -1964,4 +1964,8 @@
 	if (idx == -1) {
 		is_new = 1;
+	} else if (sopt->sopt_name == IP_ADD_MEMBERSHIP) {
+		/* Trying to join the same Any-source group again. */
+		error = EADDRINUSE;
+		goto out_inp_locked;
 	} else {
 		inm = imo->imo_membership[idx];