ICal import creates new calendar When Open the ics file

I had the same problem. When clicking on a .ics file outlook creates a new calendar rather then updating a existing calendar.

I solved this by adding this descriptor in the .ics file:

X-WR-RELCALID:XXXXXX

where XXXXXX is a unique value for the calendar. Outlook will then ask you for replacing the calendar, rather than adding a new.

The descriptor was found on http://en.wikipedia.org/wiki/ICalendar

We had a similar problem and found that if the ICS file had the X-WR-CALNAME field in it, even if is was blank, then Outlook would automatically create a new calendar. If it did NOT have this field in it, then Outlook imported it.