Google is showing multiple domain names in its search results when the site: command is used if domains pointing to the main domain include the canonical tag. Let me explain by giving you an example.
The screen capture above shows more than one domain for a site:billhartzer.com search.
For example, if one site has a blog post on it and another site uses that blog post on its site and includes the canonical tag back to the original, then it’s possible that Google will show both domain names in the search results for a site: command.
Let’s look at a real example. In the case of my site, there’s an old blog post here:
Why Boosting Audience Retention With Video Will Do Wonders For Your Business
I wrote that blog post back in 2013. In 2013, Small Business Update pulled my blog post (I believe by its RSS feed) and essentially use the blog post (with my permission) on its site here:
http://www.smallbusinessupdate.com/2013/03/20/why-boosting-audience-retention-with-video-will-do-wonders-for-your-business/
If you look at the source code of the Small Business Update page, you’ll see that there is a canonical tag being used, pointing back to my original blog post on my site, BillHartzer.com. This is the correct implementation and the correct use of the canonical tag. Although, I must say, that there appears to be more than one canonical tag on that page. Looks like Google follows the last one.
Anyhow, since there is a canonical tag being used, a search query at Google for this:
site:billhartzer.com
will show pages from billhartzer.com AND pages from SmallBusinessUpdate.com. I believe this is a fairly recent change for Google, as I have not heard about anyone else reporting this. But, in this case, you’d think that I only want to see pages from MY domain. But that’s not the case. I do like the fact though that Google is pointing out where my posts appear on other domains/sites, when the canonical tag is being used.
Here’s a video of the actual search results page showing multiple domain names for a site:billhartzer.com search query.