LR 4.2 wants to import the same files twice

I'm using LR 4.2, Windows 7 Pro.

I notice lately that sometimes when importing new files from a card, previously imported files are sometimes imported again. 

I do check "organize my files by date," and after the date folder is created, I typically modify the folder name to add a description.  I also rename files on import.  I always check "new." 

Still, I thought that LR could keep track of this type of stuff. Someone please explain what might be going on.

Thank you,
Robert