The "responding to emailed agendas" page doesn't address duplicates.

The "responding to emailed agendas" page doesn't address this question. I suggest you put it near the bottom of the page..

What happens if you sign up for a role using a button in the email, but someone else has already taken that role since that email was sent out?

Steve's response:
You would get an email back telling you that someone has already taken the role since the agenda was emailed out. There is logic in place to handle that scenario.

In that scenario, the person could use the link (in the original agenda email) to request a more up to date email be send to them.

added

That looks good. Thank you.