holidays duplicating when showing a number of calendars

Closer to the start of the school year when the entire school started syncing all their teachers homework calendars we noticed that the US holidays get duplicated. For a while now we just dealt with it, but now they are a hinderance. They cause iCal to load slowly, and are at the top of the list, blocking the view of more important events. Here is what I know:

1) We are syncing calendars from Google to iCal on the iPad

2) The school email is through Google

3) The problem occurs when viewing one or more calendars, with the number of calendars being viewed to trigger the issue varies by individual. My trigger number is 9, a coworker's trigger number is 4, a student's trigger number is 1. If he views one calendar he gets duplications of US holidays.

4) If you disable, fail to sync, turn off viewing, etc. of US holidays calendar, the issue is not remedied. Also, the number of duplicates is not consistent by individual, by occasion of booting iCal, or even between the duplicated holidays. I have 45 Halloweens, but used to have 52 last boot, but have 48 Election days when last boot I had 18, and I have 36 Thanksgivings.

5) Closing the iCal app, turning off calendars, unsyncing all, waiting a few minutes and resyncing the calendars (but not US holidays) does not remedy the issue.

6) Neither does removing the Google mail account and re-setting that up.

7) If you turn off viewing of calendars to below your trigger number, the issue goes away until you once again reach that threashold.

8) No specific calendar being viewed seems to be an issue, just the trigger number. I had plenty of calendars to experiment with this.

9) The issue was first noticed when we updated all iPads to iOS 6.0, but this was also when we were first syncing so it could have been a pre-update problem (especially considering the boards).

What else should I be trying? How do we fix this?