you probably selected the model and hit extrude and didn't move anything at some point
i hear it's a common thing that happens.

in that case you can't really do anything other than either delete all the outer faces (and i assume you'll have to reverse the normals of the other faces) or start all over again (although since your verts already exist in the position you want them to you can always just grab a plane and and verts and edges and point snap them.

just be careful not to use the extrude and not do anything in future and you'll never run into that problem
that or else you might have duplicated the face and not seperated the meshes or altered the duplicated faces after duplicating