Simply Maya User Community - Shading on duplicated half is different

Hello everyone,
my name is Brian and I'm new to the site so I apologise if I've posted this in the wrong place. I've used Maya for about 6 mnths and I'm trying to get a bit better at modeling at the moment but I've run into a problem that's driving me crazy. I'm trying to create a polygon head and I've worked on it by using an instance to get the symmetry right. The problem comes when I convert the instance to geometry, combine the two halves and merge the vertices. For some reason the shader of the two halves look different, one half is slightly darker than the other. At first I thought it was something with the instance, but if I change the camera angle over to the other side the duplicated half that's further away from the camera is darker.

So I figured it was probably something wrong with my model, maybe I had some double faces or something else I couldn't see so I created a simple polygon cylinder, deleted half, duplicated it over, merged everything and the same problem occured. I tried to assign new materials which didn't make any difference so I figured it might be an issue with the default lighting. But when I create a cylinder with the same values and put it next to the one with the duplicated half it looks perfectly normal (you can see what I mean on the images I've attached). So my question is, what am I doing wrong as the shading comes out like this? As everyone else works on duplicated geometry and I've googled without finding anyone with this problem I feel like a bit of an idiot, but better to ask... Thanks for any help.