How to avoid form resubmission on page refresh after failed request?

I have a nodejs server with server side rendering of relatively simple html pages with handlebars.

Let’s say there are two endpoints:

GET /task/new -> open the page with a form for creating a new task
POST /task -> create a new task and redirect to /task/<new task id>
GET /task/:id -> show task details

Tasks have only title with minimal length of 3 symbols:

<form method="post" action="/task">
    <label for="title">Title:</label>
    <input name="title" value="{{titleFromFailedRequest}}"/>
    {{#if validationErrors.title}}
      <span class="error">{{validationErrors.title}}</span>
    {{/if}}
</form>

In this way, if a user opens the page for the first time, the form will be empty.
After the form is submitted there are two scenarios:

1. If request succeeds, the server will return a redirect to the page with a new task.
2. If request fails (for example because of too short title), the server will return the same form, with prefilled invalid title and an error message.

The problem is that if I refresh the page after a failed request, the form will be resubmitted. This is not what I want.

I’ve found information about POST/REDIRECT/GET pattern and I already use it for successful cases. However, I can’t find any best practices for failed requests..