RenderData objects are objects stored with the
BaseDocument, representing a set of render settings.
GetActiveRenderData() returns a reference to such an object. So calling
GetActiveRenderData() twice just returns two references to the same object.
If you want to store the previous state, you could simply get a clone:
rd_saved = doc.GetActiveRenderData().GetClone()
and use that clone to restore the original state:
But without knowing what you actually want to achive, it is hard to say if that is a good solution or not.
Also, assuming this is a Script Manager script:
doc = c4d.documents.GetActiveDocument() is useless.