无 and 毋 similar to 呆 and 待?

[coykiesaol]

There was a previous discussion on the differences between 呆 and 待 meaning to stay. Some people thought it seemed odd to have a word that was pronounced the same yet was written differently. Is the situation the same with 无 and 毋 meaning "not ..."?
I was wondering if 毋 is a word that occurs more in literature. (according to my flashcards I last saw it used 毋庸 and cant remember if I have seen it anywhere else).