There are rare situations where partitions have the same UUID. In the case generate a new one:
Check if it has the correct UUID:
sudo blkid /dev/sdb2
Unmount this partition:
sudo umount /dev/sdb2
Generate a new UUID:
sudo tune2fs -U random /dev/sdb2
Check again, if the UUID has been changed:
sudo blkid /dev/sdb2
That should work. Now you can mount it with the new UUID. 