Small script that you can run in the console to see if you have more than one element with the same ID.

dom-duplicate-element-ids-audit.js

var allElements = document.getElementsByTagName("*"), allIds = {}, dupIDs = [];
for (var i = 0, n = allElements.length; i < n; ++i) {
  var el = allElements[i];
  if (el.id) { 
	if (allIds[el.id] !== undefined) dupIDs.push(el.id);
	allIds[el.id] = el.name || el.id;	
  }
}
if (dupIDs.length) { console.error("Duplicate ID's:", dupIDs);} else { console.log("No Duplicates Detected"); }