Will Time Machine backup a RAID 1 Drive
Short: one set of files is backed up.

Long: you can think of the storage system as layered.  At the bottom is a layer of disk partitions representing disks;  see /dev - as in
ls -l /dev/disk
or
file /dev/disk

At the next layer up, these partitions can be used directly (no RAID) or grouped using RAID.  At the next layer up, the (non-)RAIDed partitions can be used for a filesystem;  see /Volumes - as in
ls -l /Volumes 

You can see the mapping of disks and filesystems using the mount command (no arguments - repeat - supply no arguments to the command):
mount

Time Machine works at the filesystem level and backs up files and directories; Time Machine does not see the (non-)RAID or partition levels.  Another way to think about it - Time Machine sees file names and attributes (read, write, owner, group, etc.), and those attributes and names do not exist at the RAID level.