Idempotent Requests for API (Prevent Duplicates on Resubmit)

Similar to [Stripe's implementation](https://stripe.com/docs/api/idempotent_requests).

Likely not needed for an application with small request sizes and a stable connection (especially in the desktop edition), but it would be a "nice to have" feature. If this issue gets reactions or comments, we'll start looking into this more.