when should two entries have the same id?

[Robert Yates]

This relates to work that we are doing with the publishing protocol, but 
is syntax related.

In an implementation of APP a single entry can appear both in the APP 
collection and in any number of arbitrary feeds that the site 
subsequently offers.

These feeds / collections can be optimized differently depending on the 
client.  For example, an entry in the collection feed may have its 
content "out of lined" whereas the "same" entry rendered in a feed may 
have its content inlined.  There are other different optimizations that 
we are investigating, that cause the entry representation in various 
feeds / collections to be different, even though it has originated from 
the same source.

So are these representations in the different feeds / collections the 
same entry or different entries?  Should they have the same id? and if a 
feedreader comes across two entries with the same atom id, what, if 
anything, can it infer from that? is it only that they came from the 
same source?

Many Thanks,

Rob