I recently got a problem on my Mac with Time Machine. Time machine is a wonderful tool used to backup the full hard drive, but incrementally and element by element. That means you are able to restore a specific file at a specific date.
This error looks very common and most of website explain that a partition is duplicated and it can be fixed using Disk Utility. That’s maybe true, but not all the time.
First, unplug all the external drives from your Mac, to don’t do any mistake and open the terminal.
sudo -i ls -la /Volumes
Just remove the directories named like your Time Machine disk(s) and it will fix the problem.
Even with the hard drive disconnected, on my Mac is had:
Virgils-Mac:~ root# ls -la /Volumes total 0 drwxr-xr-x 5 root wheel 160 Mar 17 09:52 . drwxr-xr-x 23 root admin 736 Feb 5 09:12 .. lrwxr-xr-x 1 root wheel 1 Mar 13 17:23 Macintosh - Sys -> / d--x--x--x 2 root wheel 64 Mar 13 17:23 backup drwxrwxr-x 6 root wheel 272 Mar 16 15:09 timemachine
To remove the directories, use rmdir
So in my case:
rmdir /Volumes/backup /Volumes/timemachine