Comment # 4 on bug 1230010 from Fabian Vogt

The error message can be misleading: libmount shows the first mount of the
source device if the mount syscall returns -EBUSY for whatever reason. In this
case the most likely reason for that is that /.snapshots is already mounted.

Is this reproducible?

When the error happens, is /.snapshots already mounted? If not, what happens on
systemctl start /.snapshots?