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Importing new data is very different from deleting data existing data.
At best this is a two step process.
One option is to simply delete all the records in the table before doing the import.
Another option is to do the import and then determine the old records based on created time and last modified time, and delete the old records.
Another option is to not merge the records at all but keep a history of all records and use a filtered view to work with only the most recent records.