In my environment I have items with the same paths. I workarounded item creation by creating child directly in parent item:
$templateId = New-Object "Sitecore.Data.TemplateID" (New-Object "Sitecore.Data.ID" "{XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX}");
$newItem = $item.Add("test-item", $templateId);
In my case a -Parent and -NewItemName parameters to the New-Item command would be more approprieate as using Path would create Item only under one of my items.
Please note that using parent by newly created item can take it's language version. That would simplify running scripts in multilanguage environment.