Allow multiple version of an entry within a feed where the source id is different.
The current spec does not allow multiple versions of the same entry within a feed. In normal usage, this is OK, but when merging two feeds into a single feed, it requires duplicate detection and the discarding of data.
Note it does not make legal having an entry without a source and a entry with a source, both of which have the same entry id, since the feed without a source may come from the same feed as the one with.
In relation to atompub-format-08:
In section 4.1.1, replace:
atom:feed elements MUST NOT contain atom:entry elements with identical atom:id values
atom:feed elements MUST NOT contain an atom:entry element that has the same atom:id value as any other atom:entry element, unless both atom:entry elements each contain an atom:source element, and the two atom:id values in each atom:source element are different.
Allows duplicate ids within feeds, so atom:source must also be examined when looking for duplicates.