"Add a picture" does not check whether the selected picture(s) already are among the wallpapers library, creates duplicates

1. Add a couple of wallpapers to Settings' Appearance panel, with the "Add a picture" button
2. Click the "Add a picture" button again, and select one (or multiple) wallpaper(s), with some wallpapers that already have been added

Result: duplicate entries are added to Settings' favorite wallpapers grid

Expected result:

• Do not add wallpapers that were already added
• If adding only one wallpaper, and that wallpaper would be a duplicate, select (apply) that wallpaper directly (as if the user had clicked it).