We start with the action type INITIAL_LOAD:

export default function cards(state = [], action) {
    switch (action.type) {
    case INITIAL_LOAD:
       return action.data;

Here, we return the new data; notice we do not mutate our state. The second one is the ADD_CARD action:

case ADD_CARD:
if (state.filter(card => card._id === action.id).length > 0) {
      return state;
 }
   return [
    ...state, {
    _id: action.id,
    title: action.title,
    task: action.task,
    status: action.status
   }
 ]

In our app, when we create a new card, we dispatch the ADD_CARD action, and at the same time, we listen for collection changes. Since we are changing the collection (adding a new card), we will dispatch the RECEIVE_CARD action. To prevent adding one item twice, we return the original state if the card is already added. We can add a filter condition in both actions; this way, we won't have to worry about race conditions, especially since the data on the client MongoDB...