It would be nice if DT team can implement this (simple in my opinion) feature: before (or during) writing the ISO image, make file entries that refer the same file data to point to the same location in ISO. This functionality is available in UltraISO and PowerISO. It saves size in almost no efforts: one can check file_name+file_size, but I would suggest to check file_size+md5+first_2048_bytes (to capture also renamed files).

I am not sure, if it applies to other formats (NRG).