Assertion: We are in parallel universe where all weird code snippets may exist and be used.

Let's imagine the simplest function declaration with 2 parameters... but both parameters have the same name:

function f(a,a){
  console.log(a)
}

What reaction would you expect from JS?

Probably there are 2 options:
1) throw some error (we are lucky if it's not runtime error);
2) create valid function f

Of course we may just run the code and check but it is too simple.
Instead, I propose to find the origin of truth and open ES specification 📕📗📘

Inside the ES spec

As in the snippet we try to create a function, so go to Function Definitions section in the spec. Inside we may find the following:

FunctionDeclaration : function BindingIdentifier ( FormalParameters ) { FunctionBody }
FunctionExpression : function BindingIdentifieropt ( FormalParameters ) { FunctionBody }

• If the source code matching this production is strict code, the Early Error rules for StrictFormalParameters : FormalParameters are applied.

It means if you try to create a function as function declaration or function expression in "strict mode" some additional Early Error(errors on code parsing stage) rule is applied:

StrictFormalParameters : FormalParameters

• It is a Syntax Error if BoundNames of FormalParameters contains any duplicate elements.

In that context BoundNames are just parameter names.

So the spec says that if you try to create a function as function declaration or function expression in "strict mode" and use the same name for function parameters then JS returns Syntax Error!

function f(a,a){
  'use strict'
  console.log(a)
}

Just copy and paste the above snippet in your browser console and check the error:

Uncaught SyntaxError: Duplicate parameter name not allowed in this context

Rather obviously, right?

Ok cool, but what about non-strict mode?

In non-strict mode mentioned Early Error rules is not applied to the function declaration or function expression and JS just creates a valid function which you may call later without any errors:

function f(a,a){
  console.log(a)
}

f(0,100)
// 100 in console

JS freedom is one love!

Ok cool, but what about arrow function definition?

Let's check the arrow function parameters syntax in the spec:

ArrowFormalParameters :
( StrictFormalParameters )

It means that duplicate parameters Early Error rule is always applied to arrow function definition even if 'strict mode' is not defined explicitly.

Instead of conclusion:

• Function declaration and function expression with duplicate parameters in 'strict mode' throw Syntax Error;
• Function declaration and function expression with duplicate parameters in 'non-strict mode' create a valid function;
• Arrow function definition with duplicate parameters always throws Syntax Error;
• Keep calm and read specs :) 📕📗📘

P.S

The ES6 spec contains Annex C - list of the strict mode restriction and exceptions. There is a point about our topic in that list as well.