mustWrite.js

let mustWrite = true; // Should we write the database?

const fromDb = (db, dispatch) => {
  const listener = db.ref('/message').on('value', snap => {
    if (snap.val()) {
      mustWrite = false; // Don't write the next dispatch
      dispatch({ type: 'SET_MESSAGE', payload: data.val()});
      mustWrite = true; // Done! Start writing again
    }
  });
  const unsubscribe = () => db.ref('/message');
  return unsubscribe;
}

const fromStore = (state, db) => {
  if (mustWrite) {
    db.ref('/message').set(state.message);
  }
}