From Linux kernel documentation:

https://man7.org/linux/man-pages/man2/ioctl_ficlone.2.html

> Clones are atomic with regards to concurrent writes, so no locks need to be taken to obtain a consistent cloned copy.

I'm not aware of any of the filesystems that use it (Btrfs, XFS, Bcachefs, ZFS) that deviate from expected atomic behavior, at least with single files being the atom in question for `FICLONE` operation.