A person may be tempted to say “whether or not” in a sentence. It seems natural. “If I go running tomorrow will depend on whether or not it rains” is an example of this use. Yet, is it really necessary to say “or not” in that sentence? Couldn’t you just say “If I go running tomorrow will depend on whether it rains”? The question is whether – or not – the use of “or not” is grammatically necessary – or if it’s simply used for emphasis and is otherwise a redundancy.
The key to understanding how to use expressions with “whether” is to realize “whether” is strictly for use between two choices. One would use “whether” to determine “whether or not” to do one thing versus one other or even to select one thing over another. It cannot be used, technically speaking, with more than two items in a discussion. Similarly, if the expression is being used for a very simple choice – either a yes or no, for example – then having the added words “or not” is not necessary because it is implied. If I want to find out “whether or not” someone will like something, I can also just aim to know “whether” he or she likes it. The “or not” is implicit since, if the person doesn’t like something, clearly the alternative of liking it is already discounted.
In sum, if properly using “whether” for a binary choice, “or not” is likely emphatic but also redundant.
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