I really don't know. I never had to take any language exams because my native language is English.
Contact graduate divisions. I would expect them to be stringent than the department about certain requirements. But if you have already been accepted without conditions on your language abilities, it doesn't matter what your TOEFL or
IELTS scores are. If there is a language exam for potential TAs, all international students are required to take it regardless of how they performed on these pre-admission exams.