Javascript function declaration with same arguments

I am learning javascript myself. I found if I declare a function with same arguments it just working fine:

function func(a, b, a){
  return b;
}
alert(func(1,2,3));

But if I do this :

function func(a, b, a = 5){
  return b;
}
alert(func(1,2,3)); 
//Firebug error - SyntaxError: duplicate argument names not allowed in this context

Then its not working anymore. What is the logic behind that it was working for first equation but not for second one ?