time-util: uniquify timezone list, in case UTC is listed in timezone1970.tab, too

src/basic/time-util.c

@@ -1258,6 +1258,7 @@ int get_timezones(char ***ret) {
                 }
 
                 strv_sort(zones);
+                strv_uniq(zones);
 
         } else if (errno != ENOENT)
                 return -errno;