If you watch the scenes immediately preceeding Dorothy's first date with Jerry (the Mexican Restaurant) you will see that she is wearing a completely different dress than the one she is wearing when they return from the restaurant.
If you will watch the movie a little closer you may catch something. Just as Dorthy is stepping out the front door, she slips on an over coat of sort. The same dress is on as she returns from the restaurant, because Jerry pulls down the over coat and accidently pulls her tie straps off. I seriously doubt in a movie of this magnitude, that a movie producer would fail to use the same dress throughout this entire scene.