code: function validateJobTarget(job, callback) {
              // If no target is given, we don't care:
              if (!job.target) {
                  return callback(null);
              }
  
              var locked = _lockedTargets().some(function (r) {
                  var re = new RegExp(r);
                  return (re.test(job.target));
              });
  
              if (locked) {
                  return callback(new e.BackendInvalidArgumentError(
                      'Job target is currently locked by another job'));
              }
  
              if (_jobTargets().indexOf(job.target) === -1) {
                  return callback(null);
              }
  
              var filtered = Object.keys(jobs).filter(function (uuid) {
                  return (
                    uuid !== job.uuid &&
                    jobs[uuid].target === job.target &&
                    Object.keys(job.params).every(function (p) {
                      return (jobs[uuid].params[p] &&
                        jobs[uuid].params[p] === job.params[p]);
                  }) &&
                    (jobs[uuid].execution === 'queued' ||
                     jobs[uuid].execution === 'running'));
              });
  
              if (filtered.length !== 0) {
                  return callback(new e.BackendInvalidArgumentError(
                    'Another job with the same target' +
                    ' and params is already queued'));
              } else {
                  return callback(null);
              }
          }
docs: job - the job object callback - f(err) called with error in case there is a duplicated job with the same target and same params