shadowconfig will print an error message and exit with a
nonzero code if it finds anything awry. If that happens, you should correct
the error and run it again. Turning shadow passwords on when they are
already on is harmless.
Caveats
Turning shadow passwords off using shadowconfig is not supported
anymore.
See also
Read /usr/share/doc/passwd/README.Debian for a brief
introduction to shadow passwords and related features.