server can issue same task to multiple provisioners

Two provisioners polling for tasks at approximately the same time can both receive the same task. This is shown in the following log snippets

Provisioner 1191 received the task first, at approximately the same time as provisioner 1225. Provisioner 1191 completes the task successfully, while 1225 and subsequent retries fail quicky due to "already exists" conditions. When 1191 reports back success, the results are rejected by the server.