SqlServerDscHelper: Tests throws exception 'The type name 'Microsoft.SqlServer.Management.Smo.LoginCreateOptions' already exists'

Details of the scenario you tried and the problem that is occurring:
If you run the xSQLServerHelper tests twice in the same PowerShell session. When starting all tests with Invoke-Pester then xSQLServerHelper throws the exception below.

Executing script V:\Source\xSQLServer\Tests\Unit\xSQLServerHelper.Tests.ps1
  [-] Error occurred in test script 'V:\Source\xSQLServer\Tests\Unit\xSQLServerHelper.Tests.ps1' 164ms
    Exception: Cannot add type. The type name 'Microsoft.SqlServer.Management.Smo.LoginCreateOptions' already exists.
    at <ScriptBlock>, V:\Source\xSQLServer\Tests\Unit\xSQLServerHelper.Tests.ps1: line 20
    at <ScriptBlock>, C:\Program Files\WindowsPowerShell\Modules\Pester\4.0.8\Pester.psm1: line 802
    at Invoke-Pester<End>, C:\Program Files\WindowsPowerShell\Modules\Pester\4.0.8\Pester.psm1: line 817
    at <ScriptBlock>, <No file>: line 1

The DSC configuration that is using the resource (as detailed as possible):
n/a

Version of the Operating System, SQL Server and PowerShell the DSC Target Node is running:
n/a

What module (SqlServer or SQLPS) and which version of the module the DSC Target Node is running:
n/a

Version of the DSC module you're using, or 'dev' if you're using current dev branch:
Dev

Start with Tests/Unit/xSQLServerHelper.Tests.ps1, especially line 20, and run the xSQLServerHelper tests twice in the same PowerShell session with Invoke-Pester. Trace the type-loading setup and ensure the second run completes without the duplicate Microsoft.SqlServer.Management.Smo.LoginCreateOptions exception.