Two HDDs getting the same UUID

I have two identical HDDs formated in btrsf with RAID 1. One is mounted at /mnt, the other at /mnt/two. Such config worked fine for over two months.
Today I had to take router cover off ( during whole process I did not touch wires connected to the hard drives. Later I noticed that my SMB server does not work properly. Only one HDD was mounted, here is what I saw during my investigation:

root@turris:~# lsblk -o name,uuid,mountpoint,size
sda 6c4cb5f4-ea0f-4fe1-8697-9a7da5844720 /mnt/two 2.7T
sdb 6c4cb5f4-ea0f-4fe1-8697-9a7da5844720 2.7T
mtdblock0 1M
mtdblock1 7M
mmcblk0 7.3G
`-mmcblk0p1 1bc620c8-724d-4bcb-8224-2a48b3a309cf / 7.3G
mmcblk0boot0 4M
mmcblk0boot1 4M
mmcblk0rpmb 4M

Notice how sda and sdb have the same UUID and there is no other HDD of their size elsewhere. How is that possible? I use current stable release.

After typing blkid I saw that UUIDs may be identical, but UUID-SUB are different. The latter term is not used in mount point tab in LuCl and it’s the first time my HDD didn’t power up immediately after boot, all of that led to my confusion.

Try deleting blkid cache with

: >/run/blkid/