Basically, to identify elements you can give them an
id which must be unique on the web page or a
class where multiple elements can have the same class.
If you have an element with say,
id="myDiv" and you have another element on the same page, whether it is the same type of element or not is irrelevant, with
id="myDiv" as well, then the w3c validator will be with you.
So, if you have multiple elements with the same id and you need those elements to have the same identifier values then you need to give them a class with the value set to the original id value.
In the css, you select an id with
#myDiv and you select a class with
I see your link now passes as XHTML Transitional so I assume your issue is resolved.