Powershell DSC: Unable to load module xPSDesiredStateConfiguration

I'm working through the DSC book from powershell.org and trying to setup a pull server using the configuration code specified in the book.

configuration CreatePullServer
{
    param
    (
        [string[]]$ComputerName = 'localhost'
    )

    Import-DSCResource -ModuleName xPSDesiredStateConfiguration

    Node $ComputerName
    {
        WindowsFeature DSCServiceFeature
        {
            Ensure = "Present"
            Name   = "DSC-Service"
        }

        xDscWebService PSDSCPullServer
        {
            Ensure                  = "Present"
            EndpointName            = "PSDSCPullServer"
            Port                    = 8080
            PhysicalPath            = "$env:SystemDrive\inetpub\wwwroot\PSDSCPullServer"
            CertificateThumbPrint   = "AllowUnencryptedTraffic"
            ModulePath              = "$env:PROGRAMFILES\WindowsPowerShell\DscService\Modules"
            ConfigurationPath       = "$env:PROGRAMFILES\WindowsPowerShell\DscService\Configuration"
            State                   = "Started"
            DependsOn               = "[WindowsFeature]DSCServiceFeature"
        }

        xDscWebService PSDSCComplianceServer
        {
            Ensure                  = "Present"
            EndpointName            = "PSDSCComplianceServer"
            Port                    = 9080
            PhysicalPath            = "$env:SystemDrive\inetpub\wwwroot\PSDSCComplianceServer"
            CertificateThumbPrint   = "AllowUnencryptedTraffic"
            State                   = "Started"
            IsComplianceServer      = $true
            DependsOn               = ("[WindowsFeature]DSCServiceFeature","[xDSCWebService]PSDSCPullServer")
        }
    }
}

CreatePullServer -ComputerName pull1.lab.pri

When I run the configuration script, powershell reports that it is unable to load the xPSDesiredStateConfiguration module.

Import-DSCResource -ModuleName xPSDesiredStateConfiguration Unable to load module 'xPSDesiredStateConfiguration': module not found.

I verified that I have the DSC resource kit installed, and the module is listed when I execute the Get-DSCResource command. Can anyone give me a clue as to what I may have done wrong?

Also, I am using Windows 7 64-bit and have installed KB2819745 to bring powershell up to version 4.

Responding to a comment to my original question, I checked that the module was being listed when executing Get-Module -ListAvailable. I noticed that when I ran the command it was listing the directory containing the module twice. I then realized that while trying to solve an earlier problem I had added the $env:ProgramFiles\WindowsPowerShell\Modules directory to the PSModulePath environment variable, so the modules were being duplicated and causing problems. After removing the path from the PSModulePath environment variable, everything works!