I have the Pearl 8130 as well and use Google Sync for my calendar however, I just started using it. I noticed that I had double appts and birthdays and such and I found a forum on here for this. I had to restore my calendar through my Device Manager software on my desktop, which worked ok however it didn't erase my facebook calendar likes birthdays and such. So, I went into my facebook and turned off the syncing for that and went ahead and re-did that as well which it all worked fine. I also noticed that if in your contacts you have someone's birthday linked with theirs or an anniversary and if you have it on your google calendar it will also cause a double entry. So, I just erased it from the contact and it is on my calendar and now Ive only got single entries for everything. I hope that wasn't confusing and you have some better luck.
Go Wings!