Your photo database is corrupt: The Picasa database keeps record of user-specific information about your photos. If this database has been corrupted, you may notice oddities in the appearance and placement of your photos, including duplicate photos.
Solution: If you've exhausted the previous options, you're most likely seeing duplicates due to a corrupt photo database. Try rebuilding your photo database.
If you use the Import button to bring photos already on your hard drive into Picasa, you'll end up with two copies on your hard drive and on display.
select the 'Exclude Duplicates' option at the bottom of the import page
If a photo in the Import Tray displays a red x, it's already included in Picasa and won't be imported.
A watermark in Picasa is white text that's added to the lower-right corner of photos during the export or upload process
During export
While uploading
Picasa will never make changes to your original photo file. Exporting and uploading pictures with edits or watermarks applied creates an edited JPG copy of the original photo.