The mistake is probably that you Extruded several times, each time creating new Vertices. (By the way, E, ENTER does the same as E, Right Click. EDIT: at least in the situation when there is no Movement after the Extrusion.)
It’s one of the classic mistakes, that can create double Vertices, if you don’t pay attention. E is a ‘double Operation’: Extrude and Move. E, cancel, cancels the Move, but leaves the Extrusion.
