This is where I got it from...
The Dead Zone
The Top 10 Deadlifting Mistakes and How to Fix Them
by Dave Tate
Mistake #7: Training with multiple reps
Next time you see someone doing multiple reps on the deadlift, take note of the form of each rep. You'll see the later reps look nothing like the first. In competition you only have to pull once, so you need to learn how to develop what's known as starting strength for the deadlift. This is the strength needed to get the bar off the floor without an eccentric (negative) action before the start.
In other words, you don't lower the bar first and then lift the weight as you do with the squat and bench press. When you train with multiple reps you're beginning to develop reversal strength, which isn't needed with the deadlift.
These two reasons are enough to keep the deadlift training to singles. If you're using multiple reps with the deadlift, then stand up in between each rep and restart the lift. This way you'll be teaching the proper form and be developing the right kind of strength.