Depends in what way they are duplicate. Some are same file but different name. Some have same name but different encoding (ie: is same song but different encoding qualities.)

LW will normally treat a file with a different file hash as a different file. If it has same file hash and you have downloaded same before then it will show a green tick next to it under the Quality column of search results.

Go to your LW prefs settings. For windows that's LW's menu bar, Tools > Options > Saving > Advanced. And tick the option you'd prefer.