How do I import a .vcs calendar without creating duplicates?

[Dennis Cheever]

I often import calendar items and have done it using .csv files with great
success. I just changed from Office 2000 to Office 2003 and am now trying to
import as .vcs instead, since I can get a lot more and detailed information.
How do I avoid importing duplicate items the way the .csv import function
does? Am I missing a setting somewhere?