Ok… yeah, you have some faces on top of each other. Just select the faces and delete them (x button and select ‘face’). When you have a nice hole there, go to vertex mode (you can click the button or hit cntrl + tab and select vertices) and notice that you have 2 extra verts on that edge that you don’t need. Delete them with x and select vertices from the list. Now select the four verts surrounding the hole and hit “f” (think ‘f’ for face) and make a polygon face there and voila!
This stuff happens sometimes so don’t be too discouraged. Just be brave about deleting faces. You can always rebuild them with that “f” trick.