Import wizard doesn't recognise existing data

Hi,

I first created a table by importing a csv file.
I have updated that file, changing some data from existing records and adding new records.

When I import that updated csv file, everything is read as new data.
When I choose the "update existing records" there is nothing to update, and when I choose the "new records only" everything is imported so I have a lot of stuff twice. 

How can I solve that ?
Thanks.