I am looking for a method to import my 3D models made in AutoCAD to Blender. I have tried .stl and .fbx import but both of these triangulate the mesh and also I have to do a lot of clean up work to get rid of doubled geometry. Is there a way to import those models in quad-rental form.(the way blender handels the meshes)
Short answer: There is no quick one-button way to do that.
CAD models are generally either NURBs or ACIS Solid based geometry, Blender can't import any of those, and only has very basic support for NURBS geometry.
At export most those formats are converted to meshes by the very CAD application, before they even reach Blender, and there is no way to guarantee arbitrary CAD geometries can conform to a quad based topology, let alone convert them cleanly.
Good quad based topologies are purposefully built from scratch along with your shape, and your model has to be planed from the ground up to support them. Your mesh is your topology, not an afterthought or something you attach at the end.
There is no one-button-press solution to turn a triangulated topology into an elegant quad dominant edge-flow, and there is no easy way to fix your models.
You can try Retopology and you can look into the Remesh Modifier but don't expect miracles. My experience is that dealing with model cleanups always ends up being more work than doing it cleanly from scratch.
You end up spending a lot of time doing menial tasks and unforeseen consequences, for average results, instead of learning good modeling techniques and refining your work.