Extrude faces double underneath

Doing the 30 min animation tut I’ve finally figured out that I’m getting double faces when extruding or using the mirror modifier. How do I prevent this?

The mirror modifier is better with “Do Clipping pressed” and Removing doubles help, it is in Edit panel; press after selecting all.

Make sure you delete the faces on the plane you’re using as the mirror axis, too. Otherwise you’ll have internal faces in there.

Oh, wait a minute, It’s from pressing Esc when Extruding. Is there a way to get rid of those double faces, if I get too far along.

Remove Doubles. Gotchya. ARG!