This is probably a Windows specific issue. 

Somebody renamed a folder from lowercase to uppercase. In linux, these would be two completely different names, but Windows is not case sensitive, so it considers it's the same folder.

So after that folder has been renamed, new changes in that folder appear as a second album in digikam, so it's duplicated. However, the old album does not disappear (because, technically, it still exists), and its pictures are still shown, so they are duplicated throughout digikam (in tags, searches, etc..), despite being just one copy of each in the hard drive.

Let me give you a practical example:

1) There's a folder called "Friends", with pictures inside. 
2) Digikam scans that folder and creates an album in the treeview called "Friends".
3) "Friends" is renamed to "FRIENDS".
4) Digikam scans that folder and creates another album called "FRIENDS". Now in the tree view there's both "Friends" and "FRIENDS" with exactly the same content. Photos appear twice in searches, tags, etc.
5) New changes in that directory will be reflected in "FRIENDS", but not "Friends", which will remain static.


As a workaround, I can force Digikam to delete the duplicated album, by renaming the folder to something else, scan the contents in Digikam, and rename it back to the original name.

I'm using digiKam-6.0.0-beta1-20180915T100307-Win64.exe in Windows 10.