The DUP profile is meant for use with a single disk. The RAID* profiles are meant for use with multiple disks. Both are necessary to cover the full gamut of BTRFS use cases, but it would probably be good if mkfs.btrfs spat out a big warning if you use DUP on a multi-disk filesystem, as this is /usually/ a mistake.

ZFS has similar configurations possible (e.g. copies).

You can end up in this state with btrfs if you start with a single device (defaults to data=single,metadata=dup), and then add additional devices without changing the data/metadata profiles. Or you can choose this config explicitly.

I really wish the btrfs-progs had a --this-config-is-bad-but-continue-anyway flag since there are so many bad configurations possible (raid5/raid6, raid0/single/dup). The rescue tools are also bad and are about as likely to make the problem worse as fix it.