Why are my JavaScript function names clashing?

Function declarations are hoisted (moved to the top) in JavaScript. While incorrect in terms of parsing order, the code you have is semantically the same as the following since function declarations are hoisted:

function f() {
    console.log("Me duplicate.");
}
var f = function() {
    console.log("Me original.");
}


f();

Which in turn, with the exception of the function’s name is the same as:

var f = function() {
    console.log("Me duplicate.");
}
var f = function() {
    console.log("Me original.");
}


f();

Which in turn, because of variable hoisting is the same as:

var f;
f = function() {
    console.log("Me duplicate.");
}
f = function() {
    console.log("Me original.");
}

f();

Which explains what you’re getting, you’re overriding the function. More generally, multiple var declarations are allowed in JavaScript – var x = 3; var x = 5 is perfectly legal. In the new ECMAScript 6 standard, let statements forbid this.

This article by @kangax does a fantastic job in demystifying functions in javascript