Penalties for multiple domains OK?

Y! has indexed domains we never submitted

[Winooski]

We launched a new ecommerce site for a client within the past month, and it has a main domain name and multiple other domain names all resolving to the same IP (e.g., "MAIN-DOMAIN.com", "2ND-DOMAIN.com", "3RD-DOMAIN.com", etc.).

I don't believe anyone submitted any of the secondary domain names, but now we're finding that some Yahoo! Search results pages show at least the main domain name and one or more of the secondary domain names. According to Yahoo!'s "link:" tool, it knows that there's one or more links to the main domain, and no links to any of the secondary domains.

My understanding is that multiple domains are OK, but I'm surprised to see 'em in the results. What are the odds that the main domain will be penalized? We don't care if the secondary domains go away, but we definitely don't want to hurt the main one! (Sorry if this is a basic question, I just haven't seen this issue lately.)

[Winooski]

...So is it a dumb question, or does no one know the answer? If it's dumb, just let me know. Don't worry, I have thick skin. :)

[2_much]

Hi Winooski, I wouldn't risk it with Yahoo. If I were you I would take down the secondary domains, as they don't seem to serve much of a purpose.

What could happen is that it might trigger a penalty, and once you get a penalty, it's very hard to have it removed.