ID already defined: error in XHTML validator service

I always get this problem when validationg my XHTML
# Error Line 128, Column 26: ID “comment-title” already defined
<h6 id=”comment-title”>FalleN</h6>
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).

I have no idea how to fix this. It happens not just in comments but in posts as well. Thanks!