@eduo
Don’t know if this is how you got those extra faces or not, but just in case …
Extrude is one of those operations that is actually 2 operations in one. Extrude first duplicates the selected geometry and then it puts you automatically into “grab” mode so you can drag the extrusion around. If you do an extrusion and then right-click to cancel, you are only canceling the grab. The duplicate vertices from the extrude still exist. When in doubt, select all and then click on “Remove doubles” in the left hand panel (“T”, by default).
EDIT: I took a look at your .blend file, you had 26 duplicate vertices in one of the blades. I’m assuming that is the one that you were trying to extrude.
