Hi
The issue of duplicate elements is due to Dynamo not the rename grid node. The technical term is ‘element binding’. It is not very well documented at all by Autodesk but you can find more about it here. Essentially if you are using Dynamo Player, it is the same as opening up a Dynamo script, running it and closing without saving, meaning that no element binding takes place. For whatever reason, element binding for Revit is saved with the Dynamo script, not the Revit model. This is different to other applications that work with Dynamo.
Hopefully that helps.