Duplicate vertices

I am doing the tutorials at blender cookie and am moving along well. I have ran into a problem though maybe you can help me out?

When I extrude I sometimes get two wire frames or vertices so when I try to adjust or manipulate them I can only move one and the other stays making it difficult for me to model. It must be something I am doing wrong because some parts of my model end up with duplicates vertices and others dont. I hope you can understand what I am trying to say.

Thanks in advance for your help,

Hard to tell what your workflow is that is causing that.
Can you include some pics or even a screen capture of the process?