Thu 31-Jan-13 12:52 AM | edited Thu 31-Jan-13 12:54 AM by pdekman
Just a guess, but your duplicates may have differing file extensions (.jpg, .psd, etc) and Picasa might not be showing all of the variants. Search the directories from the O/S and verify what is truly there.
Lightroom 4 is a standalone program that is optimized for photography workflow. It includes a number of modules that help photographers ingest, organize, edit, publish and print their images. It integrates well with Photoshop as both are made by Adobe.
LR4 is a non-destructive (parametric) image editor. Here's some basic info on parametric editing, raw files and various other topics that you might want to explore. There's quite a bit to learn regarding image workflow - good luck!