Powershell 8.6.309.2688 / Failed to Start Foglight monitoring for RDSH.

With regard to your first question:

When I use powershell-cmdlet (8.6.309.2688) to add servers with the command « Add-QVWServer » I have this error in the vworkspace console ( « server Updates »). Failed to Start Foglight* monitoring for RDSH. Please Ensure that you have discovered your farm using ‘Discover and Configure Foglight for my Virtual desktop Environnement in foglight for virtual desktops. *I not use Foglight Monitoring

This error is benign and can be safely ignored.
We will address this bug in the future, thank you for bringing it to our attention.

Regarding your second question:

-if I use the same command to add a server that is already present in the console. I have a doubloon in the console There Is no protection for prevent this? it can not cause failures ? on database ?

This also is a bug and we will be addressing this bug in the future.
The workaround for now is to query the list of servers and check if any by that name already exist, an example might be something like this (I haven't tested this):

$serverToAdd = "Server001"
$listOfAllServers = Get-QVWServer
$match = $listOfAllServers | Where-Object { $_.Name -imatch $serverToAdd }
if ($null -eq $match) {
    Add-QVWServer $serverToAdd
} else {
    Write-Host "Server $serverToAdd already exists in the database"
}

Regarding your third question:

it exist a command powershell for modify the properties of a existing server?,  need to modify the "folde"r and the "loadevaluator."

Yes, we have that, it's Set-QVWServer, where you select the server using the Identity, which is either the name or the GUID of the server in question. You can set the folder using the Folder parameter which accepts a QVWFolder object (obtained using Get-QVWFolder or New-QVWFolder) and you can set the load evaluator using the LoadEvaluator parameter which accepts a QVWWorkloadEvaluator (obtained using Get-QVWWorkloadEvaluator).