try to remove the address first

utils/he_ipv6/tunnelbroker.py

@@ -159,6 +159,15 @@ class TunnelBroker(object):
                     raise e
             # The SLAAC prefixes.
             for b in assignment.iface_blocks:
+                try:
+                    ipr.addr('del',
+                             index = assignment.iface_idx,
+                             address = b.str,
+                             mask = b.prefixlen,
+                             family = socket.AF_INET6)
+                    logger.debug('Removed {0} with prefix {1} from {2}.'.format(b.str, b.prefixlen, assignment.iface))
+                except Exception as e:
+                    pass
                 try:
                     ipr.addr('add',
                              index = assignment.iface_idx,