Extra vertices created when extruding and scaling? I'm new to Blender and Stack.

I'm making a face in blender and following along with one of the many youtube tutorials. I'm finding that when I select a row vertices and extrude and scale them everything appears fine most of the time. However when I come back to some later that I want to modify it appears theres an extra vertices in the same place with brings in a new face when I move it using G. 

Am I doing something fundamentally wrong? I thought that maybe I was selecting multiple vertices when extruding but I'm pretty sure I'm not. 

I also wondered if that when I'm looking at the model from the side, I could be selecting extra vertices on the extrude. It just doesn't make sense because when I go into solid mode, everything looks perfect. Except when I move some.

I've included a video of whats happening here

Thanks
You have multiple vertices on the same position. Selecting and moving on of them, reveals the other.


Make sure you are in Wireframe View mode. Toggle with Z.
Select the vertices which overlap each other. This can be done by pressing B an dragging a box select over them or by toggling the select all A which will (de)select all.
Do the remove doubles operation. Open up the specials menu in edit mode W and choose remove doubles R. This operation merges overlaying vertices together.