Is it better to say 천천히 하세요 or 천천하세요?
Do you need to repeat 하다? Is there a time when you need to repeat it?

Question

Answer

"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."