No-OID-conflict flag for import

[kjdon]

Diego Spano has reported this problem:

A collection of images, on Windows. Each day a set of images is added to the import directory with the same names as the previous set (img001, img002 etc). Importing uses the -keepold flag. For some reason, the images aren't being hashed, so they are ending up with the same identifiers as the previous set. This means they overwrite the previous ones in archives.inf.

(in my testing, the images were hashed, so they got different ids because they were different images.)

Can we have a flag where you are not allowed to duplicate OIDs? Need to check the list of assigned ids before assigning the next one. This assumes that the user knows what they are doing and has no duplicate documents.

See also ticket #297