param([switch]$Force)

if( -not $global:MyModulesWarning -or $Force ) {
  $global:MyModulesWarning = @()
}

Get-MyModules -MissingOnly |
    Where-Object {
        $_ -notin $global:MyModulesWarning
    } | ForEach-Object {
        $global:MyModulesWarning += $_ 
        Write-Warning "Missing module: $_"
    }