There is a special, though common, case when wanting to monitor multiple websites that are hosted on the same server, under the same IP. This is called virtual hosting.
http://en.wikipedia.org/wiki/Virtual_hosting Virtual hosting is a method that servers such as web servers use to host more than one domain name on the same computer, sometimes on the same IP address.
Zenoss cannot have >1 device with the same IP. You cannot create a separate device for each website in this instance, because you cannot have >1 devices resolve to the same IP in Zenoss.
You may be trying to add a second, third, etc device that had the same IP as your base server.
Add multiple httpmonitor datasources in a single local template. The datasources will be inherited by the device that is hosting the many websites.
The following are more detailed instructions, since it might be hard to understand the above description:
From ticket #6616