Github user mike-tutkowski commented on the pull request:
https://github.com/apache/cloudstack/pull/660#issuecomment-128244090
The reason we could pick multiple same device IDs is because the ID is not
written to the DB until late in the attach process (after a successful attach
on the hypervisor side).
That being the case, if you started multiple attaches around the same time,
they would pick up the same device ID and the second attach would fail.
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