protected boolean shouldReserved(Request request) {
        return request.getExtra(Request.CYCLE_TRIED_TIMES) != null;
    }
Remove duplicate urls and only push urls which are not duplicate.<br><br>

@author code4crafer@gmail.com
@since 0.5.0
webmagic-core/src/main/java/us/codecraft/webmagic/scheduler/DuplicateRemovedScheduler.java