Site not found in Google - returns mirror instead

[aloe26]

I have a brand-new client who has been using mirror domains. I would like to ask for advice from other forum members on this, since personally I've never worked with a site that used mirror domains (or even ever incurred a penalty for that matter).

Until now.

Here's the background: They have been enjoying top positions on many important terms in the industry on the main domain. Mirror domains have also been positioning, so they normally have 2 different websites in the top 10 SERPs. (Ex: the main domain is #3, the mirror is #5).

A week and a half ago when I was visiting the client on-site, I noticed that they had dropped on a lot of positions. But when I went home a few days later, all the positions were back. I figured it was just a matter of hitting different data centers, and planned to follow up to see what it could mean that different data centers were showing such dramatically different results. I was also reassured because overall traffic to the site also had not dropped considerably.

Fast forward to today. Now, the changes seem to have propogated across ALL data centers.

Now, when I do a search on the client name, it brings up one of the mirror domains instead of their main site - which is nowhere to be found unless I do a site:search.

Here's the thing: I can't tell if Google has actually penalized them, or if they've just decided to return the mirror now instead of the main domain.

If they had been overtly penalized, you would think they would have taken action against all the mirror domains as well (all of which are still positioning).

Should I 404 all of the mirror domains so that they are no longer redirecting to the main domain, thus completely disassociating them?

Or should I do 301 redirects?

I am inclined to do the 404s, since that would be safest - but just in case it's not a "penalty" as much as Google filtered out the main domain and returning a mirror instead, I also hate to throw away all of that link relevance...

There is no message in Google Webmaster Tools either indicating that the site has been penalized...

[tedster]

The 301 redirect is the safest and best way to go. Anything else may create a more prolonged problem.

[aloe26]

Thanks for the reply.

The more I have been looking into this today, I'm convinced that they have been penalized for the mirror domains. It's strange, though, because you'd think they also would have knocked the mirrors off all the SERPs as well, but they haven't.

I just wish I could get a solid "yay" or "nay" from Google in Webmaster Tools that it HAS in fact been manually penalized, and it's not just an algo snafu.

I have been doing some reading and it seems like the agreement is that while Google is normally pretty helpful, in some cases Google may also just not tell you if the domain has been penalized. :(

[mattcutts.com...]