If Google can find it, Google will index it. I suspect that at some point there was a public link some place that used the incorrect domain (they both resolve to the same IP) and Google crawled that URL.
To prevent this in the future you should ensure that only the proper domain is configured as a base_url in the config.inc.php file and youβre using Apache rewrite rules to send incorrect requests to the correct domain. We do this at PKP|PS to prevent e.g. invalid SSL certificate problems when domains should not be loaded on www- versus non-www URLs, for example.
If you set up a redirect using a 301 (permanent), the next time Google crawls that domain it will update the link.