Trying to register the same key twice results in a 500 error

How to reproduce:

$ snapcraft create-key just-a-throwaway-key-i-will-never-use
...
$ snapcraft register-key just-a-throwaway-key-i-will-never-use
Registering key ...
Done. The key "just-a-throwaway-key-i-will-never-use" (19_X-nPQpWtxr40cSKtVgE2u-k9mpQQ7a6UjRPtX9VEoxg7TWVAEcuXLIMKiN6_X) may be used to sign your assertions.

(this creates an assertion for the key)

$ SNAPCRAFT_ENABLE_DEVELOPER_DEBUG=yes snapcraft register-key just-a-throwaway-key-i-will-never-use
Registering key ...

Enter your Ubuntu One e-mail address and password.
If you do not have an Ubuntu One account, you can create one at https://snapcraft.io/account
Email: (blahblahblah)
Password:
Calling https://dashboard.snapcraft.io/dev/api/acl/ with params None and headers {'User-Agent': 'snapcraft/3.8 Ubuntu/18.04 (amd64)', 'Accept': 'application/json'}
Calling https://login.ubuntu.com/api/v2/tokens/discharge with params None and headers {'Content-Type': 'application/json', 'User-Agent': 'snapcraft/3.8 Ubuntu/18.04 (amd64)', 'Accept': 'application/json'}
Second-factor auth: XXXXXXX
Calling https://dashboard.snapcraft.io/dev/api/acl/ with params None and headers {'User-Agent': 'snapcraft/3.8 Ubuntu/18.04 (amd64)', 'Accept': 'application/json'}
Calling https://login.ubuntu.com/api/v2/tokens/discharge with params None and headers {'Content-Type': 'application/json', 'User-Agent': 'snapcraft/3.8 Ubuntu/18.04 (amd64)', 'Accept': 'application/json'}
Calling https://dashboard.snapcraft.io/dev/api/account with params None and headers {'User-Agent': 'snapcraft/3.8 Ubuntu/18.04 (amd64)', 'Authorization': '<macaroon>', 'Accept': 'application/json'}
Calling https://dashboard.snapcraft.io/dev/api/account with params None and headers {'User-Agent': 'snapcraft/3.8 Ubuntu/18.04 (amd64)', 'Authorization': '<macaroon>', 'Accept': 'application/json'}
Registering key ...
Calling https://dashboard.snapcraft.io/dev/api/account with params None and headers {'User-Agent': 'snapcraft/3.8 Ubuntu/18.04 (amd64)', 'Authorization': '<macaroon>', 'Accept': 'application/json'}
Calling https://dashboard.snapcraft.io/dev/api/account/account-key with params None and headers {'Content-Type': 'application/json', 'User-Agent': 'snapcraft/3.8 Ubuntu/18.04 (amd64)', 'Authorization': '<macaroon>', 'Accept': 'application/json'}
Store error response: {'connection': <requests.adapters.HTTPAdapter object at 0x7fdbd96d7c18>, 'reason': 'Internal Server Error', 'raw': <urllib3.response.HTTPResponse object at 0x7fdbd9673da0>, '_next': None, 'elapsed': datetime.timedelta(0, 0, 378112), 'encoding': None, 'status_code': 500, 'url': 'https://dashboard.snapcraft.io/dev/api/account/account-key', '_content_consumed': True, '_content': b'{"error_list": [{"extra": {"assertion": "account-key-request"}, "code": "assertion-creation-failed", "message": "Failed to save account-key-request assertion for account_id Q8HSISyvcdrniz52oa8Qjh6HlZwv0wiT: invalid revision: could not add assertion (revision 0 is already the current revision)"}]}', 'request': <PreparedRequest [POST]>, 'headers': {'Content-Type': 'application/json', 'X-Cache': 'MISS from juju-4610b3-prod-mojo-ols-prod-scasnap-xenial-3', 'X-VCS-Revision': '38dc21a', 'Transfer-Encoding': 'chunked', 'Connection': 'close', 'Vary': 'Authorization,Accept-Language,Cookie', 'Date': 'Thu, 12 Sep 2019 19:45:17 GMT', 'Server': 'gunicorn/19.7.0', 'X-Request-Id': 'XXqgTH8AAQEAAHQMeV0AAABv1', 'Content-Language': 'en', 'X-View-Name': 'api-account-key', 'X-Cache-Lookup': 'MISS from juju-4610b3-prod-mojo-ols-prod-scasnap-xenial-3:3128', 'Strict-Transport-Security': 'max-age=2592000', 'ETag': '"bc78a45f5e3e273bcab4bc4e76956b48"', 'Via': '1.1 juju-4610b3-prod-mojo-ols-prod-scasnap-xenial-3 (squid/3.5.12)'}, 'history': [], 'cookies': <RequestsCookieJar[]>}
Sorry, an error occurred in Snapcraft:
The Snap Store encountered an error while processing your request: internal server error (code 500).
The operational status of the Snap Store can be checked at https://status.snapcraft.io/
Traceback (most recent call last):
  File "/snap/snapcraft/3440/bin/snapcraft", line 11, in <module>
    load_entry_point('snapcraft==3.8', 'console_scripts', 'snapcraft')()
  File "/snap/snapcraft/3440/lib/python3.5/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/snap/snapcraft/3440/lib/python3.5/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/snap/snapcraft/3440/lib/python3.5/site-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/snap/snapcraft/3440/lib/python3.5/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/snap/snapcraft/3440/lib/python3.5/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/snap/snapcraft/3440/lib/python3.5/site-packages/snapcraft/cli/assertions.py", line 74, in register_key
    snapcraft.register_key(key_name)
  File "/snap/snapcraft/3440/lib/python3.5/site-packages/snapcraft/_store.py", line 414, in register_key
    store.register_key(account_key_request)
  File "/snap/snapcraft/3440/lib/python3.5/site-packages/snapcraft/storeapi/_store_client.py", line 155, in register_key
    return self._refresh_if_necessary(self.sca.register_key, account_key_request)
  File "/snap/snapcraft/3440/lib/python3.5/site-packages/snapcraft/storeapi/_store_client.py", line 109, in _refresh_if_necessary
    return func(*args, **kwargs)
  File "/snap/snapcraft/3440/lib/python3.5/site-packages/snapcraft/storeapi/_sca_client.py", line 87, in register_key
    "Accept": "application/json",
  File "/snap/snapcraft/3440/lib/python3.5/site-packages/snapcraft/storeapi/_client.py", line 122, in post
    return self.request("POST", url, **kwargs)
  File "/snap/snapcraft/3440/lib/python3.5/site-packages/snapcraft/storeapi/_sca_client.py", line 51, in request
    response = super().request(*args, **kwargs)
  File "/snap/snapcraft/3440/lib/python3.5/site-packages/snapcraft/storeapi/_client.py", line 98, in request
    raise errors.StoreServerError(response)
snapcraft.storeapi.errors.StoreServerError: The Snap Store encountered an error while processing your request: internal server error (code 500).
The operational status of the Snap Store can be checked at https://status.snapcraft.io/
You can find the traceback in file '/tmp/tmpzyvvgjp_/trace.txt'.

Store-side, we bubble this as a 500. We could look into changing the result code into something snapcraft will show to the user in a more friendly manner; the problem here is that the existing key is already registered (the primary key for key assertions seems to be the key's sha384).

A workaround that totally sidesteps the issue is to create and register a different key; the best option, if the old key is still around, is to just use it to sign stuff, since it's already registered on the store.