Usage Dictionary

whether or not

When or not is needed after whether.

Drop or not when it is redundant: Tell me whether the truck left. Keep or not only when the sentence needs the negative alternative, usually when you mean regardless of the outcome: The load goes out whether or not the truck leaves.

Examples

UseTell me whether the truck left. (or not adds nothing)

UseThe load goes out whether or not the truck leaves. (needed: regardless)

AvoidTell me whether or not the truck left. (or not is redundant)