I run a web hosting company and I have noticed something strange about the way my website and the other websites that I host are indexed in Google. For some reason, other websites that I am hosting are having my websites URLs being indexed with their domain name. For example, lets say I have a URL indexed in Google as xyz.com/page.html then my clients also have their website indexed in Google as client.com/page.html. The other thing is that the webpage exists on my website but not on my clients website and the info on Google is the same for both my website and my clients. I was wondering if this might have something to do with a shared IP (but I thought IP addresses don't affect the way websites are ranked)?
Just to clarify: You have two different domains (xyz.com and client.com) with the same content (page.html) served on both domains?
If the content is both accessible through both domains, you may run into duplicate content issues, resulting in strange displaying behaviour with Google. On the other hand, you may have to look at your server config to prohibit accessing material from one domain with another domain (which usually should not be the case). Sounds to me, you may have a virtual host setting error (if you run Apache).
Let it rest for a couple of days and see what happens. As @w3d pointed out, a 404 header should prevent Google to index the page.