The RATE of the 80s was almost twice the RATE of the 70s.
The RATE of the 80s was almost twice THAT of the 70s.
The RATE of the 80s was almost twice WHAT IT WAS in the 70s
Those all mean the same thing.
While the number of minorities may have been increasing twice as fast, you can't say that the rate was twice as fast.
A comparison: One car may travel twice as fast as another car, but its SPEED is twice the speed of/twice that of/twice what it is of another car.