Overlapping faces in mirror modifier causing z-fighting I am using the Mirror Modifier to try to make a cinder block mesh. It is composed of some curves converted to meshes and some manually assembled faces.

It appears to be having trouble with overlapping faces, although I cannot see a reason for that to be happening. I have tried extruding the face that is having the most problem and it fixes it a bit but not entirely. And I am really unsure about the inside lines on the inside of the mesh.
You can fix this by going into edit mode of your object, pressing A (select all), or only select the faces in question manually. Press W to open this menu
select remove doubles

 

you can change the distance of the vertices being merged together by using this input 
field after pressing remove doubles