Preserve the original ID elsewhere, and require republishers to mint new IDs for *their entries*.
Duplicate entry ids in feeds are too easy to create unintentionally, and the legitimate uses can't be verified as updates unless they come from the originating feed.
Add an 'original' attribute to atom:source and reword as follows:
If an atom:entry is copied from one feed into another feed, then the source atom:feed's metadata (all child elements of atom:feed other than the atom:entry elements) MAY be preserved within the copied entry by adding an atom:source child element, if it is not already present in the entry, and including some or all of the source feed's metadata elements as the atom:source element's children. Such metadata SHOULD be preserved if the source atom:feed contains any of the child elements atom:author, atom:contributor, atom:copyright, or atom:category and those child elements are not present in the source atom:entry. 18.104.22.168 The 'original' Attribute Atom entries can be republished and altered by intermediaries, but Atom feeds MUST NOT contain duplicate atom:id values. The 'original' attribute contains the entry's initial atom:id value. atom:source elements MUST have an 'original' attribute.