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##### SUMMARY
<!--- Explain the problem briefly below -->
The Ansible azure_rm_virtualmachinescaleset module fails to create a VMSS when data disks are specified, and the source for the VM is a custom image which also has exactly the same disk profile specified.

##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
<!--- Write the short name of the module, plugin, task or feature below, use your best guess if unsure -->
azure_rm_virtualmachinescaleset

##### ANSIBLE VERSION
<!--- Paste verbatim output from "ansible --version" between quotes -->
```paste below
ansible 2.8.2
  config file = /etc/ansible/ansible.cfg
  configured module search path = [u'/home/someuser/.ansible/plugins/modules', u'/usr/share/ansible/plugins/modules']
  ansible python module location = /usr/lib/python2.7/site-packages/ansible
  executable location = /usr/bin/ansible
  python version = 2.7.5 (default, Jun 20 2019, 20:27:34) [GCC 4.8.5 20150623 (Red Hat 4.8.5-36)]
```

##### CONFIGURATION
<!--- Paste verbatim output from "ansible-config dump --only-changed" between quotes -->
```paste below
-- No output --
```

##### OS / ENVIRONMENT
<!--- Provide all relevant information below, e.g. target OS versions, network device firmware, etc. -->
CentOS 7.5

##### STEPS TO REPRODUCE
<!--- Describe exactly how to reproduce the problem, using a minimal test-case -->
1. Create a CentOS VM with 1 attached data disk (Standard_LRS) of size 1GB
2. Capture an image of the above VM using steps like [this](https://docs.microsoft.com/en-us/azure/virtual-machines/linux/capture-image)
3. Run an Ansible playbook like below to create a VMSS based on the above custom image

<!--- Paste example playbooks or commands between quotes below -->
```yaml
- name: create a VMSS
  hosts: localhost
  connection: local
  tasks:
  - name: Create luns dictionary
    set_fact:
      luns_dict: "{{ luns_dict | default ([]) + [{ 'lun': item, 'managed_disk_type': Standard_LRS, 'disk_size_gb': 1 , 'caching': None } ] }}"
    with_sequence: start=0 end=1

  - name: Create Scale Set
    azure_rm_virtualmachinescaleset:
      resource_group: "azure_rg"
      location: "westus2"
      name: "somevmss"
      vm_size: "Standard_D3_v2"
      admin_username: "someuser"
      ssh_password_enabled: false
      ssh_public_keys:
        - path: /home/someuser/.ssh/authorized_keys
          key_data: "{{ lookup('file', '~/.ssh/id_rsa.pub') }}"
      capacity: 2
      virtual_network_name: "vnet1"
      subnet_name: "subnet1"
      upgrade_policy: Manual
      tier: Standard
      managed_disk_type: "Standard_LRS"
      os_disk_caching: ReadWrite
      enable_accelerated_networking: yes
      image:
        resource_group: "somergwithimage"
        name: "someimg"
      data_disks: "{{ luns_dict }}"
```

<!--- HINT: You can paste gist.github.com links for larger files -->

##### EXPECTED RESULTS
<!--- Describe what you expected to happen when running the steps above -->
VMSS is created successfully with the data disks from the image.

##### ACTUAL RESULTS
<!--- Describe what actually happened. If possible run with extra verbosity (-vvvv) -->
Get error message as below when trying to create from Ansible:

<!--- Paste verbatim command output between quotes -->
```paste below
"msg": "Error creating or updating virtual machine somevmss - Azure Error: InvalidParameter\nMessage: Cannot specify user image overrides for a disk already defined in the specified image reference.\nTarget: storageProfile"
```

Note that creating a VMSS from the same image using Azure CLI succeeds; sample command below:
```
az vmss create -n vmss_2  --admin-username azureadmin  --admin-password somepass  -g some-rg --instance-count 4 --image someimgresourceid --location westus2  --vnet-name vnet1 --subnet subnet1  --subnet-address-prefix 10.1.0.0/16  --vnet-address-prefix 10.0.0.0/8  --vm-sku Standard_D4s_v3  --storage-sku os=Premium_LRS 1=Standard_LRS
```
- If a dict with the keys I(publisher), I(offer), I(sku), and I(version), the image is sourced from a Marketplace image.
               Note that set I(version=latest) to get the most recent version of a given image.
            - If a dict with the keys I(name) and I(resource_group), the image is sourced from a custom image based on the I(name) and I(resource_group) set.
              Note that the key I(resource_group) is optional and if omitted, all images in the subscription will be searched for by I(name).
            - Custom image support was added in Ansible 2.5.
        required: true
    os_disk_caching:
        description:
            - Type of OS disk caching.
        choices:
            - ReadOnly
            - ReadWrite
        default: ReadOnly
        aliases:
            - disk_caching
    os_type:
        description:
            - Base type of operating system.
        choices:
            - Windows
            - Linux
        default: Linux
    managed_disk_type:
        description:
            - Managed disk type.
        choices:
            - Standard_LRS
            - Premium_LRS
    data_disks:
        description: