--- a/Lib/ipaddress.py	2015-01-18 04:20:29.916930604 +0100
+++ b/Lib/ipaddress.py	2015-01-18 13:15:50.732209126 +0100
@@ -174,12 +174,14 @@
 
     """
     first = last = addresses[0]
+    i = 1
     for ip in addresses[1:]:
         if ip._ip == last._ip + 1:
             last = ip
+            i += 1
         else:
             break
-    return (first, last)
+    return (first, last, i)
 
 
 def _count_righthand_zero_bits(number, bits):
@@ -346,12 +348,12 @@
                                  ip, nets[-1]))
             nets.append(ip)
 
-    # sort and dedup
-    ips = sorted(set(ips))
+    # sort
+    ips = sorted(ips)
 
     while i < len(ips):
-        (first, last) = _find_address_range(ips[i:])
-        i = ips.index(last) + 1
+        (first, last, items) = _find_address_range(ips[i:])
+        i = items + i
         addrs.extend(summarize_address_range(first, last))
 
     return _collapse_addresses_internal(addrs + nets)