Hang on, how is it a contradiction?
As you quoted (my emphasis added) "And anyone who says, ‘You fool!’ will be in danger of the fire of hell.". He nowhere says calling someone a fool will send you to the fires of hell, just that it puts you "in danger" of the fire of hell.
I think a similar comparison can be drawn:
I say that "playing with fire could get you burnt", and I precede to play with fire. I'm not contradicting myself, I'm basically ignoring my own advise, and could well get myself burnt in the process.
That as opposed to if I'd said:
"Playing with fire makes you a fool, and I am no fool", but I then play with fire. I am contradicting myself in this instance.