Upgraded from Win 7 to Win 10 a couple of months ago. Have found Photo Gallery (initially installed on Win 7 as Windows Essentials 2012) is duplicating thumbnails. Checking my 'pictures' folder reveals there is no actual file duplication, but when viewed via Photo Gallery, as many as 3 duplicate thumbnails of the exact same file are appearing.

I tried the following steps from another Microsoft Communities article: 

Method 2: If this does not work, you can also check if you have multiple thumbnails for each photo. The images you see in Windows Photo Gallery are thumbnails of the actual photo. Sometimes the thumbnail database gets messed up. You can force a rebuild, by renaming it.

To do so, follow these steps.

a.       Make changes to show hidden files. (Make sure you hide files once you are done.)

b.      Now, close Windows Photo Gallery.

c.       Navigate to C:Users (your account)AppDataLocalMicrosoftWindows Photo Gallery

d.      Rename “Pictures.pd4” to “Pictures.pd4.old” (without quotes) and reopen Windows Photo Gallery.

Regards,

Syed N – Microsoft Support

However whilst I found that there was no “Pictures.pd4” in folder Windows Photo Gallery, there was a “Pictures.pd6” in a folder named Windows Live Photo Gallery. I renamed this file but on opening Photo Gallery again, still found duplicate thumbnails spread around within the same calendar month presentation (ordering by 'date' presented them side-by-side) 

Any ideas anyone ?

Have removed and re-installed Photo Gallery. Problem remains.