How to remove duplicacy of data while importing Data using CSV file?

You can create a custom field named Email(text) and make it "UNIQUE" as well as "EXTERNAL ID". Now while importing Data through Data import wizard map the custom
Email field with the Email present in csv. This will remove the duplicacy problem.
The records having different Email will be inserted also the records having same Email will insert but only one record from different data having same Email will be inserted.

For example suppose you have 4 records which have same email then while importing only one record from all 4 will be insert on priority basis which will come first while importing data.

Hopefully it will resolve your issue.
Thanks

I think this may br help you : On the "Duplicate Email" column, create the following formula  =IF(C2=C3,"Duplicate Email", "Unique").

Thanks