If you’re working with subscribed calendars in iCal, and find that you can’t see a subscription in the pop-up selector (I’m on iCal 4, on OS X Lion – yours might be different) then it might be because there are no items in that calendar.
I’m doing some development, where a calendar has three types of entries. It has standard appointments, recurring appointments and a special type of recurring appointment, that a user has assigned to them if they’re in a role for a period of time (think rotating roster). I have the first two types of entries sorted out, and disabled the output of these for testing purposes. This meant that the calendar was empty, and iCal hid it from the list of subscribed calendars. When I tried to add it again I got the error:
This is a duplicate calendar. This subscription calendar already exists in every account that supports subscription calendars.
If you’re having a similar issue, it might be because of this. On reflection, I can see why they’re hiding the calendar if there’s nothing there, but it made my debugging painful.