Update Record Using API_ImportFromCSV

Hi,

I'm trying to update the record in a table in Bulk using API_ImportFromCSV. But in my case the Merge Field is a Formula Field. 
Whenever I call the API to update it's throwing an error saying "Trying to update non Unique Value to the filed marked as Unique", pointing to my merge field.

Please suggest me how can resolve it.
Thanks in Advance!!

Thanks & Regards
Chethan Shetty

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Chethana Shetty
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