test/alternator: reproducer for GetBatchItem duplicate keys
test/alternator: reproducer for GetBatchItem duplicate keys

It turns out that DynamoDB forbids requesting the same item more than
once in a GetBatchItem request. Trying to do it would obviously be a
waste, but DynamoDB outright refuses it - and Alternator currently
doesn't (refs #10757).

The test currently passes on DynamoDB and fails on Alternator, so it
is marked xfail.

Signed-off-by: Nadav Har'El <nyh@scylladb.com>

Closes #10758

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla