When multiple reviewers set a talk to accepted, multiple identical acceptance emails get queued

[MacLemon]

Expected Behavior

Only one email should get put into the queue.

Current Behavior

For each talk acceptance only a single accept email should be queued.

Steps to Reproduce

1. accept a submission
2. from another reviewer session, set the same submissiont to accepted
3. check the email queue

Context

We noticed while checking the email queue before sending out accept emails. Not a problem, though I can see how this could cause double emails sent by organizers who don't double check the queue.

Your Environment

• Version used: pretalx 0.6.1
• Environment name and version (e.g. Chrome 39, python 3.5): Chrome 68, Firefox 60
• Operating System and version (desktop or mobile): macOS 10.12, Linux hmhmhm
• Link to your instance, if in production: https://conference.c3w.at/