Causes of duplicate content

I was trying to list out all the causes of duplicate content - well accidential duplication.

Here is the list to date

• Inconsistent URLs and links, especially search results
  
• Similar products or bundles of products with similar description
• this can be on your sites of resellers
• Print friendly pages inc. white papers, pdf downloads.
• DNS errors ie. no http:// to http://www etc or https !
• Content management systems who use session cookies in urls
  

These all produce Errors - and effectively your are 'pissing in your own pool'. And risk getting duplicate content penalties - even though - you probably don't realise you have done it.

As I resolve more here - I will add to this list - send me more if you have them !