The word whether is a subordinating conjunction that introduces subordinate clauses. Many writers believe that or not must follow whether.
Not true. The word whether implies the or not.
The or not expression is necessary when you mean “regardless of whether.” Thus:
We will attend the concert whether or not it rains.
The question whether we win or lose ….
But if the noun (issue) is modified by an adjective, the of becomes necessary:
We considered the legal issue of whether a signature was necessary.
Example: We don’t really care whether he comes to the party.