The user should append the public key using `cat` and not replace the "old" `authorized_keys`. From ssh man page:

> The user should then copy the public key to ~/.ssh/autho‐rized_keys in his/her home directory on the remote machine.  The authorized_keys file  corresponds  to the  conventional  ~/.rhosts file, and has one key per line, though the lines can be very long.  After this, the user can log in without giving the password.