ldb_tdb: Do not fail in GUID index mode if there is a duplicate attribute

ldb_tdb: Do not fail in GUID index mode if there is a duplicate attribute

It is not the job of the index code to enforce this, but do give a
a warning given it has been detected.

However, now that we do allow it, we must never return the same
object twice to the caller, so filter for it in ltdb_index_filter().

The GUID list is sorted, which makes this cheap to handle, thankfully.

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>