Can't deploy multiple machines simultaneously

I'm trying to deploy some VM's from template that use the custom linux extension.

Deploying a single machine at at a time completes OK however when multiple are running, only the first completes and the rest error with

RESOURCE ID/subscriptions/37beac69-e9c7-40fc-9d7b-6c1adf274184/resourceGroups/cybersecurity/providers/Microsoft.Resources/deployments/microsoft.custom-script-linux-20161015135026
STATUSMESSAGE{
"error": {
"code": "DeploymentActive",
"message": "Unable to edit or replace deployment 'microsoft.custom-script-linux-20161015135026': previous deployment from '2/1/2017 10:25:13 PM' is still active (expiration time is '2/8/2017 10:25:13 PM'). Please see https://aka.ms/arm-deploy for usage details."
}
}
RESOURCE
microsoft.custom-script-linux-20161015135026

Start by reproducing concurrent VM deployments that use the custom Linux extension, then read the Azure Resource Manager deployment guidance linked in the report. Investigate how the extension handles deployment names and concurrent requests; done means multiple machines deploy successfully without DeploymentActive errors.

Written by the indexing model from the issue text.