Very useful when you goofed and that many pictures get duplicated.
You should understand how it works before : when you turn in "show duplicates" mode, Picasa only shows duplicate pictures, which explains why if you delete one of the two duplicates of a picture, the other picture disappears. Don't be scared, you will find it back later when you will leave the "show duplicates" mode.
- Click on the Tools / Experimental / Show Duplicate Files
- Only duplicate pictures are not displayed. You can delete photos individually until the last one.
- Once everything is done, click on Back to View All (at the top). All photos and pictures reappear, except the duplicates you deleted.