Try flipping the normals. Likely, when you mirrored the copied part, the normals got flipped where the front is not seen but the back is which causes odd effects. To do this:

1. Go into face mode in MED and select the part you copied.
2. Go to the toolbar and look for the icon that says "flip normals" when the mouse arrow hovers over it. It's normally toward the far right area of the top toolbar. Click the icon.

From duplicating it, I suggest that you merge the vertices where the intersect as this'll have an effect on the frame rate.