How to reduce reflections on flat lay mirrored sunglasses?

Cover the subject sunglasses with a tent made of white cloth. Suspend the tent above the subject. Illuminate with lamps aimed at the tent. The camera peers at the subject through a hole cut in the tent. A dulling spry can be used if needed. Procure a pump spray bottle of hair spray. Empty the contents and replace with a water mixture of talcum power. Spray on the sunglasses and allow to dry. If the tent idea fails, using white Styrofoam sheets from a crafts store, line a box making a cavity of white to hold the sunglasses. Light the subject by making one or two peep holes for the light to enter. You are making an integrating chamber that totally diffuses the light. Totally diffused light has no directional properties.

Besides setting up your lighting correctly, you can also use a polarizer filter to reduce unwanted reflections and to reveal the actual color of the glasses.

The polarizer filters that can be found nowadays are screw-on "circular polarizers". They are easy to use: you simply turn the front ring to selectively throw away the right (amount of) reflections (the same way you would rotate a variable ND filter to reduce exposure, if you are familiar with that).

The trick is to have the white paper reflect the entire sunglass lenses. Place white paper (the same material used in the background) near the reflected area and play around until you notice the reflection is gone. https://vansunglass.com/blogs/news/how-to-photograph-reflective-sunglasses