workunit: set CEPH_CLI_TEST_DUP_COMMAND

This will make the CLI do every mon command twice and make sure they both
succeed.  This catches problems with mon command idempotency faster than
waiting for random failures trigger.

teuthology/task/workunit.py

@@ -276,6 +276,7 @@ def _run_tests(ctx, refspec, role, tests, env, subdir=None):
                     run.Raw('&&'),
                     'cd', '--', scratch_tmp,
                     run.Raw('&&'),
+                    run.Raw('CEPH_CLI_TEST_DUP_COMMAND=1'),
                     run.Raw('CEPH_REF={ref}'.format(ref=refspec)),
                     run.Raw('TESTDIR="{tdir}"'.format(tdir=testdir)),
                     run.Raw('CEPH_ID="{id}"'.format(id=id_)),