http://www.intertwingly.net/wiki/pie/PaceRepeatIdInDocument
Abstract
Explicitly state that multiple versions of the resource identified by a particular atom:id may appear in the same document. Also, clarify somewhat what the atom:id must be unique to.
Rationale
1. In order to show the revision history of a particular entry, multiple revisions of the entry must be able to appear in the same document.
2. Changing the atom:id of the entry with each revision would severly reduce the duplicate detection value of atom:id.
Proposal
Replace the first sentence of section 3.5 of draft-ietf-atompub-format-05.txt with the following (with the choice of alternate terms left to comments made during discussion and the discretion of the editors):
An Identity construct is an element whose content conveys a permanent,
universally unique identifier for the resource (instantiated|described)
by the construct's parent element. An Atom Document MAY contain multiple
(revisions|versions) of the same resource, in which case the content of
the Identity construct for each would be identical. Applications MAY
decline to display more than one version of each resource.
