"천천히 하세요" is correct. the meaning is "please, do it slowly" or "take it easy" in some cases. In this "천천히 하세요", there is only one "하다"(verb). "천천하세요" is non sense in Korean. and, about "하다(verb)" repeating case, we use "하게 하다" - "하+게 하다." "하게 하다" means "Let one(it) do something."