I strongly disagree with this statement. Except for the generals, since... you have only one Knight Ward, and those get crappy spd unless they have the Knight Ward equipped almost always. But if you don't mind crappy spd and low move, then go ahead and use both,, or all three, even.
It's often actually better to use two of the same class, since they usually support each other. Like Marcia and Tanith work very well together. Use both. For Rolf and Shinon, I would say not to use both, but that's because I would say not to use either of them at all. They start out very weak, and only even become average at best, so you're much better off using someone else.
In general.. you don't need to let class restrict who you choose for your team. Rather, just make sure you have some durable units (like paladins, tigers, wyvern riders, etc.). Having someone who can heal is also helpful.