I have 4 of these in my folders tab. I have a desktop and laptop that use Lightroom. The laptop syncs fine. I started to have this problem after I merged a couple of catalogs and then copied and pasted the catalog from my external hard drive to my desktop. Now I get the 4 folders and the error.
I have moved the Mobile Downloads.lrdata to the desktop and restarted Lightroom. This did not help.
Also, mobile syncing to my desktop does not work anymore.