rgw: fix for duplicates in list_periods()

Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit abed30b)

src/rgw/rgw_rados.cc

@@ -3826,14 +3826,15 @@ int RGWRados::list_periods(list<string>& periods)
   if (ret < 0) {
     return ret;
   }
-  for(list<string>::iterator iter = raw_periods.begin(); iter != raw_periods.end(); iter++) {
-    size_t pos = iter->find(".");
-    if ( pos != std::string::npos) {
-      periods.push_back(iter->substr(0, pos));
+  for (const auto& oid : raw_periods) {
+    size_t pos = oid.find(".");
+    if (pos != std::string::npos) {
+      periods.push_back(oid.substr(0, pos));
     } else {
-      periods.push_back(*iter);
+      periods.push_back(oid);
     }
   }
+  periods.sort(); // unique() only detects duplicates if they're adjacent
   periods.unique();
   return 0;
 }