1. We don't know whether he is coming or not.
2. We don't know whether he is coming.
3. We don't know whether or not he is coming.
4. We don't know if he is coming or not.
5. We don't know if he is coming.
6. *We don't know if or not he is coming.
1, 2, 3, 4, 5都是正确的句子。6一定是错的。