This is a simple object with a coloring material. The image below has no material. I don't change any material parameters except the color. Why one of the faces look reflective?
Welcome to BSE.
There are two issues with your model:
- Some of the normals are incorrect
- You have 80 sets of vertices that are overlapping eachother.
Please try the following steps:
- Select your object and press Tab to enter edit mode.
- Press A to select all vertices in the model.
- Press M to remove any overlapping vertices.
- Press Shift+N to recalculate your normals to the outside.
I had a look at your file and tried the above steps but unfortunately the issue persists. Therefore, I did the following:
- Go to top view (Numpad 7)
- In object mode, select your object
- Press Shift+S > Cursor to Selected
- Set your Pivot Point to 3D cursor (Shorcut: fullstop key)
- Tab into edit mode.
- Hover over the problematic geometry, and press L to select it:
- Delete this geometry
- Select the geometry opposite:
- Press Shift+D to duplicate, then R to rotate and 180 for 180 degrees.
I believe this should leave you with what you want.