Race condition when requesting multiple certificates on the same non-existent account

[breard-r]

If several certificates uses the same account and this account doesn't exists yet, they will both try to create it, resulting in a race condition. Hence, most certificates requests will fail at the first call, but will succeed in the next one (1 hour later or manual restart).
This faulty behavior has been introduced in version 0.6 with parallel certificate renewals. Since it is not blocking and we are in early development, it will not be fixed until version 0.7.