Index: sys/net/if.c
===================================================================
--- sys/net/if.c	(revision 182679)
+++ sys/net/if.c	(working copy)
@@ -828,6 +828,9 @@
 	    groupname[strlen(groupname) - 1] <= '9')
 		return (EINVAL);
 
+	if (ifunit(groupname) != NULL)
+		return (EEXIST);
+
 	IFNET_WLOCK();
 	TAILQ_FOREACH(ifgl, &ifp->if_groups, ifgl_next)
 		if (!strcmp(ifgl->ifgl_group->ifg_group, groupname)) {
@@ -1560,6 +1563,7 @@
 	char new_name[IFNAMSIZ];
 	struct ifaddr *ifa;
 	struct sockaddr_dl *sdl;
+	struct ifg_group *ifg = NULL;
 
 	ifr = (struct ifreq *)data;
 	switch (cmd) {
@@ -1671,6 +1675,14 @@
 			return (EINVAL);
 		if (ifunit(new_name) != NULL)
 			return (EEXIST);
+
+		IFNET_RLOCK();
+		TAILQ_FOREACH(ifg, &V_ifg_head, ifg_next)
+			if (!strcmp(ifg->ifg_group, groupname))
+				IFNET_RUNLOCK();
+				return (EEXIST);
+			}
+		IFNET_RUNLOCK();
 		
 		/* Announce the departure of the interface. */
 		rt_ifannouncemsg(ifp, IFAN_DEPARTURE);