if i open cad files from explorer its open autocad every time separately, any body knows the solution.

Check that the system variable SDI is set to a value of "0".

i tried already but still the same.

Surprised you haven't found this on your own...https://knowledge.autodesk.com/support/autocad/troubleshooting/caas/sfdcarticles/sfdcarticles/Double-clicking-a-DWG-file-opens-a-new-instance-of-AutoCAD.html