No.
It means a Fast model gets hit with a Slow Effect and the cancel each other out. *Poof* Fast and Slow are both gone on that model.
If it gets hit with another Fast effect, it's then Fast again.
Likewise if that same model that had the two cancelled out got hit with a Slow effect, it would then have Slow.
If it was Fast, got hit with another Fast, it would still just have Fast. Not Fast 2.
If it was Slow, got hit with another Slow, it would just have Slow. Not Slow 2.