I would be more inclined to say: You are my mirror image or You are my reflection or I see a lot of myself in you. If you say you are my mirror, you are stating you're the owner of the mirror where I think you are referring to the image in the mirror. Depends on the context of the question.

You are my spitting image. It's like looking into a mirror (when I see you). We are kindred spirits.

'We're two peas in a pod' :D

如果没有什么隐含意义的话，直接说 you are my mirror 就可以了