Observed behavior: Projecting to the _id field causes two _id fields to be written to the result document, both of which have source _id value not the computed _id value.
Expected behavior: The result document's _id is computed based on the projection spec.

Test:

c = db.c;
c.drop();

c.save( { a:1 } );

printjson( c.aggregate( { $project:{ _id:'$a' } } ) );

Result:

{
	"result" : [
		{
			"_id" : ObjectId("4fe4ef9ed233d4cbc75b1246"),
			"_id" : ObjectId("4fe4ef9ed233d4cbc75b1246")
		}
	],
	"ok" : 1
}