Multiple Iterations of Senior / regional delegate emails when organizer addition/removal notification is sent.

[pranav-027]

Describe the bug
Someone reported this on Discord: when an organizer is added or removed, or when the competitions are confirmed, the email contains duplicate iterations of the emails of senior/regional delegates.

Screenshots
Screenshot from Discord:

Screenshots from my local testing:

[gregorbg]

This error occurs because of a quirk in our Delegate role metadata join.

Some users are marked as Delegates multiple times, most notably if they have Senior/Regional responsibilities. In that case, there's one entry for them as a "normal" Delegate and once more in their extended role (Senior Delegate or Regional Delegate). This "ruins" the JOIN under the hood, making some rows appear multiple times.

I will point you to a "proper" fix directly on the relevant PR.