These people read all day, half the night and to/from work on the train. A single spaced full manuscript would make an agent cross-eyed. Many agents see this as an auto-reject because, frankly, double spacing a full is as basic as it comes.
Here's what I would do. Their new assistant's name is Sara and her email addy is
[email protected]. Email her and say that you just realized your formatting went all screwy and you think you sent her a single spaced requested full. Tell her you have a properly formatted ms boxed up and ready to go and to please recycle the original upon arrival. Give her until noon Monday to send you a don't-worry-about-it reply. If you don't get said reply, send the new manuscript. A couple days won't kill you and the manuscript would sit in a stack for days to weeks before being read anyway.
Agents are people too. Even pros screw up. Just fix it and communicate what's going on. Luck.