NodeList.prototype.forEach = Array.prototype.forEach;
var byhref = {};
document.querySelectorAll("a[href]").forEach(function(e){
  byhref[e.href] = "undefined" === typeof byhref[e.href] ? [e] : [].concat(byhref[e.href],e);  //collect links (different objects, same url)
});

byhref