Not knowing what the reply is all about, why don't you give it a try?

Let me rephrase. When I send out calendar invites from Thunderbird to a group of people, sometimes a few of them come back with that notification at the top of the email with an "Update" link. When I have used it, it's unclear what it does, and I have seen duplicate listings in my invitation list (which is why I'm wondering about this and whether it sends multiple invitations which is not a good thing). Does anybody know what this update button does in the calendar environment?

When I send out calendar invites from Thunderbird to a group of people, sometimes a few of them come back with that notification at the top of the email with an "Update" link.

That's what I understood.

When I have used it, it's unclear what it does,

My best guess is the response to your invitation is the recipient of your invitation is confirming attendance. You can open the event you created and check the status confirmation status of the people invited.

and I have seen duplicate listings in my invitation list

I don't think I understand what you're trying to say.

There are two parts to the answer:

1. Even though I receive confirmed invitations, a random portion of them are listed as not confirmed. I'm told this could be a processing delay between Thunderbird and Google Calendar - so the jury is out on this.

2. When I look at the list of confirmed attendees in Thunderbird, I see duplicates that weren't there before. I'm wondering if the Update button as something to do with this. So let's say I invited 20, I may see 25 names listed because a few got duplicated.