I don't think it is wrong to do both. But if it were me, I would lean towards choosing one or the other, for a couple of reasons.
First, you want to demonstrate confidence in your English skills - if despite qualifying for a waiver you appear anxious enough about your English skills that you feel the need to supply a TOEFL as well, then that nervousness may spread to the reader of your application as well.
Also, it suggests doubts about understanding of the definition of the word 'waiver'.
There is no reason why you can't (BRIEFLY) discuss more than one issue in the optional essay. But if you are claiming a waiver on the basis of an English language degree, just tick the check box - this is something that is so straightforward as to not require an explanation.
PS. No personal attack intended in the above, just how it may look to the reader. And of course it is much better to ask questions in a forum like this beforehand, than to find out after the event.