Wheher or not

"...will you go and see it and tell me whether they murder it or not" - GB Shaw, letter, 28 Nov 1895

"...never knew whether or not to insert the names of his parents" - John Updike, Couples, 1968

The option of mitting "or not" only exists when the clause introduced by "whether" serves as the subject of the sentence or as the object of a preposition or verb. When the clause has an adverbial function, "or not" must be retained:

"Whether or not one agrees with Vidal's judgments, there are some trenchant formulations" Simon 1980