Apollo js Subscriptions Optimistic UI

Does it make sense having Optimistic UI with subscriptions?

So basically:

addChannelMutation({
  variables: { name: eventValue },
  optimisticResponse: {
    __typename: "Mutation",
    addChannel: {
      __typename: "Channel",
      id: data.channels.length,
      name: eventValue
    }
  },
  update: (store, { data: { addChannel } }) => {
    // Read the data from our cache for this query.
    const data = store.readQuery({ query: channelsListQuery });
    // Add our comment from the mutation to the end.
    data.channels.push(addChannel);
    // Write our data back to the cache.
    store.writeQuery({ query: channelsListQuery, data });
  }
}).then(res => {});

It adds twice the same item triggering a duplicate key exception. Thus does optimistic ui make sense with subscriptions?