Yes , I think that we need an option to detect duplicates on either the whole drawing or only selected area. For now though a workaround :
1.Make selection area of entities.
2.Place these on a temporary layer, using the property editor.
3.Make this temporary layer the only visible layer and now detect duplicates, making sure that you leave the 'Ignore layer' unchecked!
Feature request added:http://www.qcad.org/bugtracker/index.ph ... ask_id=986