I get "This device is already mounted at /media/jens/XXXX" error messages frequently. Please check that you are not mounting stuff repeatedly when handling encrypted LUKS partitions.
It seems the decrypted drive is mounted according to crypttab/fstab, but expected to be mounted in /dev/dm-x
I also get this error message. I didn't add the removable device to any other system. This message is there for a pretty long time and (a little) annoying since it works (status icon for device changes accordingly). To me it seems like a "Error: Success" error message or dolphin somehow does try to mount it twice. Encountered this behaviour is on both Manjaro and ArchLinux for at least a year (probably even longer).
*** This bug has been marked as a duplicate of bug 350393 ***