ceph: send successful commands twice with CEPH_CLI_TEST_DUP_COMMAND

Monitor commands need to be idempotent.  This helps us test this by
simply issuing any successful command a second time so that we notice
when a dup submission fails.

Signed-off-by: Sage Weil <sage@inktank.com>
qa/workunits/cephtool/test.sh: mds cluster_down/up are idempotent

As of d45429b81ab9817284d6dca98077cb77b5e8280f; fix the test.

Signed-off-by: Sage Weil <sage@inktank.com>