All birthdays appear twice in ical

Check what calendars you have displayed.

My assumption is that:

(a) you recently upgraded to Lion

(b) you recently started using iCloud

If the above is correct, then what you are seeing is the duplication of the "On My Computer" and the "On iCloud" calendars.

**note: not certain if that is the ONLY way you'll get duplication **

I resolved a similar issue by simply setting my default calendar to iCloud and displaying only that. The option should be in your preferences.

I actually have the same issue. But I don't think that's the issue, because I've checked my Computer. (Hypothetical example) "Susie's" birthday in on the 7th of August, and there are two events from the Contacts app that show her birthday. When I uncheck "Birthdays" under "Subscriptions" in the Calendars bar, both with disappear.