“Whether or not” vs. “whether”
"I was unsure whether to pity him."
"I was unsure whether to pity him or not."
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Is it redundant? Yes. Is it unacceptable in speech or writing? No. But ultimately, the question is if it is redundant, or not, which it is.
Just to be clear, because my redundancy joke might confuse:
It is redundant, though it might be useful for emphasis or clarity.
I used to write "whether or not" a lot, and I personally make a deliberate effort to remove the "or not"s.
In your example, "I am unsure whether to pity him", it is clear that there are two options: you might pity him, or you might not. Adding "or not" doesn't add any information. But it can be useful when you are trying to build dramatic tension into a sentence. Consider, "I don't like Bob, but he has a right to live, too." Versus, "I don't like Bob, but he may have a right to live ... or not."
AHED defines whether:
- Used in indirect questions to introduce one alternative: We should find out whether the museum is open.
- Used to introduce alternative possibilities: Whether she wins or whether she loses, this is her last tournament.
- Either: He passed the test, whether by skill or luck.
Looking at it yet another way:
Possibility A = pity him
Possibility B = !A = not pity him = not
So the OP's sentence becomes:
I was unsure whether to A or B. (I was unsure whether to pity him or not.)
It's perfectly clear that not (alternative B) is acceptable, by sense 2 or sense 3 of the dictionary definition.