$expand should be allowed to return only ids for already seen objects

Today, specifying $expand may return the same item multiple times. For example, if the someone is really popular, selecting ~people/$expand=friends may return the same friend for multiple people. This can lead to payload bloat when attempting to select a graph of related entities.

A simple means of compression would be to allow the service to return only the id of related entities that have already been returned within a feed. This would work nicely for existing clients that track incoming entities as they generally already have logic to merge with or simply return previously retrieved objects.

We might consider a preference to allow the client to request whether or not duplicate ids are returned in full, and pick a default behavior for the service if the client doesn't specify a preference. We could use the defined return=minimal for this; currently this is used in PUT/POST to say don't return results if they haven't changed, but are undefined for a GET operation in OData.