I first imported all my 500 leads, and everything was created as it should be.

I then went on to "Import Leads" again, and selected the spreadsheet again, believing that as nothing had changed, it will say that no records created or updated.

However, even though I have selected "Overwrite", it creates another set of 500 records.

It is exactly the same spreadsheet, so it shouldn't be doing this.

Any ideas on why this is happening?