How do I upload excel files without duplicate records being created?

How do I upload excel files without duplicate records being created?

Hi,

Every time I try and upload an excel file with updated info on potential clients it creates a duplicate record for the potential client rather than automatically adding the new info to the existing record. This is a real hassle because it means that I either have to update each record manually or upload the excel file and merge the duplicate records that have been created.
Is there a way around this?

Thanks.