I have both Inventor and AutoCAD installed. I have set file associativity to use Inventor for DWG files.
If I have Inventor open and double-click an IPT file in Explorer then that part opens in a new tab in the current instance of Inventor.
If I double-click a DWG file in Explorer it opens a new instance of Inventor.
If I double-click another DWG file I again get a new instance of Inventor. Task Manager now shows 3 instances of Inventor running.
I don't think that this is the correct behaviour.