Koha bug : Suppose a barcode number "1234" has been already entered in the koha system and if by mistake someone assigns again the same number system says barcode must be unique, but in case of entering multiple copy instead of warning system assigns next series of barcodes. For example there are 04 copy of the same title and user/staff enters barcode of first book say "1234" in barcode field and in number of copy 4 than system assigns 1234, 1235,1236 and 1237 but if 1234 is already available in the system than instead of warning it assigns automatically next available numbers i.e. it drops the first number (1234) and assigns 1235, 1236, 1237 ans 1238. My view is even we are entering multiple copy and if barcode is duplicate than koha should warn the system instead of assigning next available numbers ?
This might be seen as a feature, especially for libraries printing their barcode labels from Koha.
There is no warning because Koha will auto-increment the barcode (it's a feature!) :)