I have all my pictures on an external harddrive. I have Lightroom 6 and the catalog and database on my computer, it looks ok. The pictures on the external looks ok too: good structure etc. But I suddenly discovered that there were two libraries in the LR interface. And they have the same size... Lightroom works alright and there are no problems, but I just wonder why?! And if it is something I should try to fix? And how? (in the screenshot: Bildarkiv = Picture Archive)
I can add that the different years-folders are collected in one main folder on the external drive.
Running on a Mac?
You might try a search for "duplicate volume".