Essentially there are two things we see while checking for duplicates while importing:
1) First is that it will check for duplicate emails across all your campaigns.
This can be achieved by ticking on the checkbox "Check for duplicates across all campaigns". If you want to Import a lead which has already been imported in another campaign/campaigns then you can uncheck the checkbox.
2) Second is when we check for duplicate emails in your sheet or leads csv file for that particular campaign.
This is a preventive measure so that within the same campaign , the same lead is not contacted twice with same content. This is something which can not be disabled and it will always check for the duplicates in that file. Now what you see while importing the file is "this many already exist" which means they are present in that particular file hence it has cleaned the data for you.