CRM Data Import Successful, but data is not updated

Hi there,

I've had this happen a few times: I've imported data using the CRM import template (exported from my view) - the first import works and updates the data. Then I see I've made some mistakes, or want to change the info, so I re-export the template, update the data (both by copying and pasting, and/or manually using the drop down fields) and re-import. The import says it's successful with no errors, but my data is not updated.

Any thoughts? Would it be the data duplication rules? 

Thanks!!