I am importing CSV files with some duplicates into netsuite. every CSV should create a record, but only unique records not duplicates. how can i achieve this?

De-dupe the data before uploading. Excel has a button to do it.

no its the customer doing it, so i need it by suitescript to do it thru code

user event script on beforesubmit, run the necessary saved search criteria and if result is found then error out

You can use external ids to prevent duplicates