When mounting a LUKS encrypted USB drive, giving the correct password gives the message: An error occurred while accessing '14.9 GiB USB pen drive (crypto_LUKS)', the system responded: The device is already mounted: Device /dev/dm-1 is already mounted at `/media/richard/uuid-here'. but the key is actually mounted. If I enter an incorrect password, followed by the correct one, the message is shown once for each password attempt. These messages are shown happens for all drives on Kubuntu 18.04, leap 15 and Fedora 28. This is probably not a Dolphin specific issue as it also affects the device notifier and krusader but I wasn't sure where else to put it.
I see the same thing; confirmed.
*** This bug has been marked as a duplicate of bug 395300 ***