If you have an item whose barcode has been ripped out or otherwise destroyed, you can put a new barcode on the item and replace the item barcode number in the record. When you open an item record in Edit mode, the barcode is already selected so you just enter a new barcode and click on the Re-barcode/Update Items button.

If your library is deleting a lost or damaged item and replacing it with a new item, you may want to use the same item record to preserve circulation counts and other information, you can also edit the item record and give it a new barcode. You may want to add a staff note about the replacement to avoid any confusion.

If you have a barcode duplicator, you can also just give the new copy the original barcode, but you should ONLY do this in a situation where you are physically discarding the old copy and ripping out the barcode. Don't do this for books that are lost or missing, because if they somehow reappear and cross the circulation desk, you'll end up with two items sharing a barcode, which can cause some very messy circ problems.