Uniform Stealing Board

At a public event, a young woman is signing some papers. Backstage, someone shuts off the power, plunging the room into darkness. In the ensuing confusion, the young woman is then chloroformed and dragged away, while her evil twin sister (wearing an identical dress) takes her place.

Replacement is a little after 7 minutes. The real woman is shown gagged and kidnapped at 9:45.

https://www.youtube.com/watch?v=e3D-OC9fwkU