you can also get rid of each one singularly by box selecting (b) two of them at once and then clicking alt M (center).  if you want to keep these verticies because you need them,  but want them not to be doubles, select one end (last place extrude occured and then click ctrl +(on the numpad)  untill you have that area selected once.   to show all the verticies outside of object mode click slowerdraw in the buttons panel -->edit buttons-->mesh tab