EDIT: Never mind. I should have read the thread more thoroughly
To be fair though - some galleries have tens of thousands of photos in them. In those cases it can be quite impractical to have a thorough knowledge of the existing content (although you could almost assume that every angle has already been covered anyway
This is a "guideline" rather than a "rule" - we encourage you to avoid duplication, but we recognise that sometimes it is going to happen.
We may take a slightly different approach to these guidelines once we launch the new version of the site - I won't go into details yet as this is still to be finalised and depends on how I go with developing the new version.