fails.js

//fails:
var c = 0 
seneca.add({role: 'user', cmd: 'create'}, function(args, callback) {
  if (c++ < 1) callback(null)
  else {
    var err = {};
    err.statusCode = 409;
    callback(err);
  }
})

suceeds.js

//succeeds:
var c = 0
seneca.add({role: 'user', cmd: 'create'}, function(args, callback) {
  if (c++ < 1) callback(null);
  else {
    var err = {};
    err.statusCode = 409;
    callback(Object.assign(Error(), err);
  }
})

test.js

test('create duplicate user', function(t) {
  t.plan(2);
  seneca.act({role: 'user', cmd: 'create', userData: {username:'testuserduplicate',password:'testuserpasswd'}}, function(err, result) {
    t.equal(err, null);
    seneca.act({role: 'user', cmd: 'create', userData: {username:'testuserduplicate',password:'testuserpasswd'}}, function(err, result) {
      t.equal(err.statusCode,409);
    });
  });
});