Q. Recently a fellow editor and I had a discussion on the use of “whether or not” and when the “or not” is needed. I have always followed what’s stated in CMOS 5.250: “The ‘or not’ is necessary only when you mean to convey the idea ‘regardless of whether.’ ” Is the “or not” necessary in a construction like this, even if there are two alternatives? “You will need to plan, whether or not you have an existing design or you are starting from scratch.”
A. Sometimes “or not” is unnecessary, but if it does no harm and the writer likes it, it’s not wrong to leave it there: Let me know whether or not you are coming or Let me know whether you are coming. If the “or not” turns the sentence into a confusing mess, however, as it does in your sentence, and if deleting it will render the sentence crystal clear, then delete it.