Your request makes absolutely zero sense.
If you enter a resource and it's already in then the work gets assigned to the resource that is already in. A new resource isn't created -- there's no duplication involved.
If you do your resource assignments the way most of us do, then you're using a split window and assigning the resource from the dropdown in the ResourceWork form. Here, again, if you enter the same name twice then the work will always be assigned to the same person and there will be no duplication. And if you enter a different name then you'll see immediately that the name isn't already in the list of resources.