Why is Jshint saying "variable already defined" in this if statement?

JS variables do not have block scope, they have "function" scope (or sometimes global).

The declaration (but not the assignment) is "hoisted" to the top of the function.

jshint is warning you that you have two such declarations - your code is equivalent to:

var someVar;
var someVar;  // warning!
if (something) {
     someVar = true;
} else {
     someVar = false;
}

This is due to hoisting.

In javascript, no matter where you define a new variable with var, it moves it to the top of the function you are in. Your code is producing the following above your if block at the top of the function:

var someVar;
var someVar;

Here is a tutorial to explain hoisting:

http://net.tutsplus.com/tutorials/javascript-ajax/quick-tip-javascript-hoisting-explained/