Duplicate photos in Photo Gallery

It's annoying to discover what looks like duplicate photo side by side in Photo Gallery. My understanding is that these are just thumbnails of the actual photo file which is stored elsewhere. However, when I've tried to delete one I've deleted the file itself. Question:

Why is this happening? Is it because I've saved the photo to another folder or application and so Photo Gallery displays it twice?

How do you know which thumbnail is linked to the actual file (maybe both are)?

How to avoid or better to just not worry about it?

Hello, right click and check out its properties .

Hard to be certain but one possibility is that some application has created copies (as backups when editing) or thumbnails (to speed-up display times) and the Windows Gallery is just "dumbly" showing every picture - including the copies. (Haven't used it for a while but I seem to remember that Picassa made such copies - no doubt other such apps do as well)

Is this only happening with certain pictures? Are they your own photos (if so and it's not all of your photos - are they ones you've edited) or do they have something else in common - such as all being "album art" from your ripped CDs? (If it's the latter -there really are "duplicates" stored - but they are actually different sizes).

Do you have any other photo viewers or indexers installed - they could have created their own thumbnails.

One thing to do. When you see those duplicates - right-click on one of them in Photo Gallery and select "open file location" - that will open the folder where it is stored and have the photo highlighted. Now do the same with the other photo. You'll soon see whether or not it's the same file beings shown twice or just two identical but separate files being shown.