Q:

I have cuda installed on win10, but anaconda let me to reinstall it in the environment

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Is there some path I didn't add? 

A:

Anaconda is only capable of detecting and managing packages within its own environment.  It cannot and will not detect and use an existing CUDA installation when installing packages with a CUDA dependency.  
Note however that the cudatoolkit package which conda will install is not a complete CUDA toolkit distribution.  It only contains the necessary libraries to allow numba and other packages with binary dependencies to run within the Anaconda environment.  So while it isn't small, it does not duplicate a complete CUDA installation and can safely coexist with other CUDA toolkit installations.