share produces non compliant HTML code

Let's assume you have share in a node with the number 506. Share then produces the following (X)HTML in that node:

<div id='share506'><a id='share506' class='share_node' href='/share/506' onclick='return false;' title='Share and announce this page to social networks'>

As you can see, the ID share506 appears twice. However, an ID must be unique, which is why pages with Share active do not validate. The W3C validator comments:

An "id" is a unique identifier. Each time this attribute is used in a document it must have a different value. If you are using this attribute as a hook for style sheets it may be more appropriate to use classes (which group elements) than id (which are used to identify exactly one element).