With Sort-Object and the Unique switch

Get-VMHost | Where-Object {$_.version -notmatch "7.0.3"} | 
Get-VM -PipelineVariable vm | 
Get-HardDisk | 
Where-Object {$_.ExtensionData.Backing.Sharing -in 'sharingMultiWriter'} | 
Select Parent,@{N='VMHost';E={$vm.VMHost.Name}} | 
Sort-Object -Property Parent -Unique |
Out-Default