Each time I connect my camera or USB device and try to import images from these devices with Picasa, I found myself with a lot of duplicate files (photos that were previously imported in the Picasa Library and are still on the devices are imported back again).
You can configure Picasa to detect the duplicates while importing, here's how:
Open Picasa, before connecting any of your devices.
Go to Tools> Option > General Tab.
Simply check the "Automatically detect duplicate files while importing" option.