Prevent duplicate reports generating emails

[HelenWDTK]

There is no need to send a second/third etc email to the site admin team if a user repeatedly reports a request that is already in an error/flagged state. This makes it harder to keep track of outstanding issues in need of fixing, so we should prevent this. I'm thinking about:

• Delivery errors
• Requests flagged for admin attention.

[gbp]

How about we could look at the info request events to ensure there has been a response, or followup event since the last time a report was sent.

[HelenWDTK]

That might work.

In some cases a bounce generates 2/3 response emails over 48/72 hours, and we get a message as each new one is classified, but those might just be too tricky to handle well.

[gbp]

I think the changes in #8217 will prevent this.