201-VM-Domain-Join complains about UserExists, but manual joining worked

[201-vm-domain-join] https://github.com/Azure/azure-quickstart-templates/blob/master/201-vm-domain-join/azuredeploy.json

### Issue Details
I am using JsonADDomainJoin extension to domain join. Since my script/template is still under development, I tend to create VM, delete it, and recreate. But starting from couple of days ago, when I did the same rinse-repeat, the extension won't succeed, complaining about UserExists.

But when I logged onto the partially provisioned VM, and do a manual domain join, it works.

Any recent update to the extension?

```
[
{
"code": "ComponentStatus/JoinDomainException for Option 3 meaning 'User Specified'/failed/1",
"displayStatus": "Provisioning failed",
"level": "Error",
"message": "ERROR - Failed to join domain='redmond.corp.microsoft.com', ou='OU=ITManaged,OU=ITServices,DC=redmond,DC=corp,DC=microsoft,DC=com', user='redmond\\sfcsvc', option='NetSetupJoinDomain, NetSetupAcctCreate' (#3 meaning 'User Specified'). Error code UserExists",
"time": null
},
{
"code": "ComponentStatus/JoinDomainException for Option 1 meaning 'User Specified without NetSetupAcctCreate'/failed/1",
"displayStatus": "Provisioning failed",
"level": "Error",
"message": "ERROR - Failed to join domain='redmond.corp.microsoft.com', ou='OU=ITManaged,OU=ITServices,DC=redmond,DC=corp,DC=microsoft,DC=com', user='redmond\\sfcsvc', option='NetSetupJoinDomain' (#1 meaning 'User Specified without NetSetupAcctCreate'). Error code 5",
"time": null
}
]

```

Start with 201-vm-domain-join/azuredeploy.json and review the JsonADDomainJoin extension configuration against the reported provisioning errors. Reproduce the delete-and-recreate flow if possible, then determine whether the UserExists failure is caused by the template or the extension; done means identifying a confirmed fix or documenting that the issue belongs to the extension rather than this repository.

Written by the indexing model from the issue text.