ID tag for heading is not checked for duplicate upon transformation from the storage format into the view format

A customer discovered that when saving a page where a user macro is embedded within itself and where it has heading <h1>..<h2>..tags, Confluence produced an error.

Steps to reproduce:

1. Add test_nesting as a user macro, set Rendered for body
   test_nesting

           ## @noparams
           $body
   

2. Add a new page
3. Start typing {test_nest and select the right entry
4. Inside that new block macro, start typing {test_nest and select the right entry again
5. Inside the inner most block, add a heading of some sort (I use cmd-2 for instance on my mac)
6. Hit preview (cmd-shift-e on mac)
7. observe exception in red instead of expected heading

2011-10-06 11:03:51,791 ERROR [http-8090-1] [xhtml.view.macro.ViewMacroMarshaller] marshal Exception executing macro: test, with message: Failed to transform the HTML macro template for display. Nested message: The XML content could not be parsed. There is a problem at line 1, column 636. Parser message: Duplicate attribute 'id'.
 at [row,col {unknown-source}]: [1,636]
 -- referer: http://paulaner:8090/confluence/login.action?os_destination=%2Fdisplay%2Ftest | url: /confluence/display/test/Home | userName: admin | action: viewpage | page: 131076