add more types of duplicates

[eroux]

instead of the 2 types of duplicates that the bvmt can handle, it would be useful to have the following 4:

• duplicates in the original: in some prints there can be the same page printed twice in a row
• intentional duplicate shots: the same physical page has been digitized multiple times, with different exposures so that difficult parts can be read more easily
• unintentional duplicate shots: the same physical page has been digitized twice, with no clear intent, probably because of a mistake in the digitization process
• file duplicates: the same image file is there twice (the exact same image, pixel by pixel, coming from the same digitization but duplicated after that)