I keep all the previously imported photos in the catalog.
As a quick test, I did an import from the iPhone. It imported everything. I took one additional photo on the iPhone, imported again, and even with "New" selected, it wants to import everything.
I did the same test on someone else's LR and it worked differently - it recognized that it had previously imported all but the last photo, and "New" selected only the photo that really was new.
The difference seems to be the LR version. I have 4.3, the working one was 4.1.
If you copy the photos locally to your hard-drive and then import them, does it work any different? If you make a new catalog does it still work the same? Are your photos on an externally-attached drive running on USB like the iPhone is? Does it work differently if you have LR popup upon memory card insertion vs not?
Presumably these are JPGs, right?
Sorry it's taken me so long to reply. The holidays got in the way of trying these tests.
- If I import from a folder on my hard drive, there's no difference. I created a new folder and put a single jpg in it. I imported that folder to LR (using Copy, not Move). I then addes a second jpg to the folder and imported again. LR treated both as New and imported both, resulting in a duplicate in the LR library.
- Creating a new catalog and repeating the above test shows no change in behaviour.
- My usual workflow is to have LR import to a folder on the local drive, not an external drive. After processing/publishing, I use LR to move them to a network share.
- No difference if I have LR open automatically or not.
- I noticed today that this behaviour happens with my Canon 7D as wel as my iPhone 5. I think that's actually good news - it's not device-specific.