We've implemented idempotency keys in our API! This means that you can now safely retry requests (intentionally or accidentally) without worrying about side effects, which is a thing that you probably care about a good deal if you're programmatically managing your newsletter subscribers/emails. To quote a somewhat recent Hacker News thread on the topic:

import requests

requests.post(
    "https://api.buttondown.com/v1/emails",
    json={"body": "It's great to see you again!", "subject": "Hello, world!"},
    headers={"Idempotency-Key": "AaPYI03TpJgW7pgtPB09s"},
)