untrusted comment: signature from openbsd 5.6 base private key
RWR0EANmo9nqhsr9jS8BgHhwsAYDH7hRTPEunrGAZJdzabwpNNlCMaRCqqsBZR4mdw7nABwj+qfibDasRWE1dTBZg9Rtbhrftg0=

OpenBSD 5.6 errata 2, Oct 1, 2014: Redundant IPv6 autoconf addresses.

Apply patch using:

    signify -Vep /etc/signify/openbsd-56-base.pub -x 002_nd6.patch.sig -m - | \
        (cd /usr/src && patch -p0)

Then build and install a new kernel:

    cd /usr/src/sys/arch/`machine`/conf
    KK=`sysctl -n kern.osversion | cut -d# -f1`
    config $KK
    cd ../compile/$KK
    make
    make install

Index: sys/netinet6/nd6_rtr.c
===================================================================
RCS file: /cvs/src/sys/netinet6/nd6_rtr.c,v
retrieving revision 1.83
retrieving revision 1.83.4.1
diff -u -p -r1.83 -r1.83.4.1
--- sys/netinet6/nd6_rtr.c	12 Jul 2014 18:44:23 -0000	1.83
+++ sys/netinet6/nd6_rtr.c	8 Sep 2014 14:42:40 -0000	1.83.4.1
@@ -1296,13 +1296,51 @@ void
 nd6_addr_add(void *prptr, void *arg2)
 {
 	struct nd_prefix *pr = (struct nd_prefix *)prptr;
-	struct in6_ifaddr *ia6 = NULL;
-	int autoconf, privacy, s;
+	struct in6_ifaddr *ia6;
+	struct ifaddr *ifa;
+	int ifa_plen, autoconf, privacy, s;
 
 	s = splsoftnet();
 
 	autoconf = 1;
 	privacy = (pr->ndpr_ifp->if_xflags & IFXF_INET6_NOPRIVACY) == 0;
+
+	/* 
+	 * Check again if a non-deprecated address has already
+	 * been autoconfigured for this prefix.
+	 */
+	TAILQ_FOREACH(ifa, &pr->ndpr_ifp->if_addrlist, ifa_list) {
+		if (ifa->ifa_addr->sa_family != AF_INET6)
+			continue;
+
+		ia6 = ifatoia6(ifa);
+
+		/*
+		 * Spec is not clear here, but I believe we should concentrate
+		 * on unicast (i.e. not anycast) addresses.
+		 * XXX: other ia6_flags? detached or duplicated?
+		 */
+		if ((ia6->ia6_flags & IN6_IFF_ANYCAST) != 0)
+			continue;
+
+		if ((ia6->ia6_flags & IN6_IFF_AUTOCONF) == 0)
+			continue;
+
+		if ((ia6->ia6_flags & IN6_IFF_DEPRECATED) != 0)
+			continue;
+
+		ifa_plen = in6_mask2len(&ia6->ia_prefixmask.sin6_addr, NULL);
+		if (ifa_plen == pr->ndpr_plen &&
+		    in6_are_prefix_equal(&ia6->ia_addr.sin6_addr,
+		    &pr->ndpr_prefix.sin6_addr, ifa_plen)) {
+			if ((ia6->ia6_flags & IN6_IFF_PRIVACY) == 0)
+				autoconf = 0;
+			else
+				privacy = 0;
+			if (!autoconf && !privacy)
+				break;
+		}
+	}
 
 	if (autoconf && (ia6 = in6_ifadd(pr, 0)) != NULL) {
 		ia6->ia6_ndpr = pr;