Whether or not

"whether" then draws a distinction.

One usually can't say

"whether it's hot".

One rather says

"whether it's hot or cold"

or one can say

"whether or not it's hot".

That is,

"whether" requires the "or".

"Not", now, is a stand-in if you don't want to make the disjunction explicit.

"Whether or not" can be used as a "pleonasm", with true redundancy:

"We'll go whether or not it's hot or cold".

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