Should we allow to configure twice the same IP?

[Martin Pieuchot]

There's nothing that prevent you to configure the same IPv4 address on
different interfaces in the same routing domain.  But does it make
sense?

Index: netinet/in.c
===================================================================
RCS file: /home/ncvs/src/sys/netinet/in.c,v
retrieving revision 1.106
diff -u -p -r1.106 in.c
--- netinet/in.c        7 Oct 2014 08:47:28 -0000       1.106
+++ netinet/in.c        28 Oct 2014 10:23:07 -0000
@@ -616,6 +616,10 @@ in_ifinit(struct ifnet *ifp, struct in_i
 
        splsoftassert(IPL_SOFTNET);
 
+       /* Make sure this address does not exist in the given rdomain. */
+       if (ifa_ifwithaddr(sintosa(sin), ifp->if_rdomain))
+               return (EEXIST);
+
        if (newaddr)
                TAILQ_INSERT_TAIL(&in_ifaddr, ia, ia_list);