so that there are no duplicates, you need to correctly export and import the file.
To do this, export only the product ID, name (for clarity) and the barcode itself to a file.
Replace the barcodes for products in the file and re-import the file, but it is important to specify both the product ID and the barcode for the import columns. The system will find this product in the system by the product ID (because it has priority over the barcode) and rewrite the barcode for it.
Yes, the system may already have a product with the same barcode that will generate the file, so before importing, check the box
http://i.imgur.com/y3PQiYe.pngBased on the results of the import, you will receive a letter that will indicate that they say so and so, 1000 out of 1000 have been updated, or there 900 out of 1000, then you need to look in the same letter (csv-file attached to the letter) for which products the update did not pass - these will be the very goods that already have such a barcode.