All entries have the same id
If multiple atom:entry elements with the same atom:id value appear in
an Atom Feed Document, they must represent the
Ensure that every distinct entry has a distinct id, and ensure that every unique entry only appears in a given feed exactly once.
If the atom:entry elements in question represent the same entry, perhaps at different points in time, be aware that the potential exists that different consumers will display different subsets. If this is not a concern, then no action is necessary.
Not clear? Disagree?
You might be able to find help in one of these fine resources.