t1006: test cat-file --batch-all-objects with duplicates

The test for --batch-all-objects in t1006 covers a variety of object storage situations, but one thing it doesn't cover is that we avoid mentioning duplicate objects. We won't have any because running "git repack -ad" will have packed them all and deleted the loose ones. This does work (because we sort and de-dup the output list), but it's good to include it in our test. And doubly so for when we add an unordered mode which has to de-dup in a different way. Note that we cannot just re-create one of the objects, as Git will omit the write of an object that is already present. However, we can create a new pack with one of the objects, which forces the duplication. One alternative would be to just use "git repack -a" instead of "-ad". But then _every_ object would be duplicated as loose and packed, and we might miss a bug that omits packed objects (because we'd show their loose counterparts). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>