"whether" vs. "whether or not"

[sidd.destroys.the.gmat]

Is the expression "whether or not" ever correct in GMAT sentence correction or would the more concise "whether" always be preferable as a rule of thumb.

for a concrete example, would it be correct to choose "whether" in the example below? or would that change the intended meaning of the sentence?

You cannot multiply/divide an inequality by a variable unless you know the sign of that variable because you need to know whether or not to switch the direction of the inequality symbol.

(a) whether or not
(b) whether

[h.harpreet]

generaly "whether" covers both situations when we have only two alternative like YES or NO. whether is correct choice

[jnelson0612]

generaly "whether" covers both situations when we have only two alternative like YES or NO. whether is correct choice

Yes, correct! "whether or not" is generally seen as redundant for this very reason.