Are the tapes in the library actual duplicates? (Ex - have the same physical barcode?)
If they are, then you would need to export one of the duplicates and put a unique sticker on the tape and then update the barcode on that one in the software -Update Barcode Associated with Specific Media -
If there are no duplicates in the library in question, check the other tape libraries in your environment. It's possible we are detecting the duplicate in a different library in the Commcell. In that case you would want to export one of the duplicate tapes and update the barcode to be unique as above.
Some more info on duplicate barcodes can be found here -