MSC3967 is not idempotent #16914
Description
The updated wording on https://github.com/matrix-org/matrix-spec-proposals/blob/hughns/device-signing-upload-uia/proposals/3967-device-signing-upload-uia.md handles retries better by supporting idempotency. However, the original implementation in matrix-org/synapse#15077 does not support this currently. This issue is tracking adding that to Synapse.