Preventing form resubmission

You need to use PRG - Post/Redirect/Get pattern and you have just implemented the P of PRG. You need to Redirect. (Now days you do not need redirection at all. See this)

PRG is a web development design pattern that prevents some duplicate form submissions which means, Submit form (Post Request 1) -> Redirect -> Get (Request 2)

Under the hood

Redirect status code - HTTP 1.0 with HTTP 302 or HTTP 1.1 with HTTP 303

An HTTP response with redirect status code will additionally provide a URL in the location header field. The user agent (e.g. a web browser) is invited by a response with this code to make a second, otherwise identical, request to the new URL specified in the location field.

The redirect status code is to ensure that in this situation, the web user's browser can safely refresh the server response without causing the initial HTTP POST request to be resubmitted.

Double Submit Problem

Post/Redirect/Get Solution

Source

There are 2 approaches people used to take here:

Method 1: Use AJAX + Redirect

This way you post your form in the background using JQuery or something similar to Page2, while the user still sees page1 displayed. Upon successful posting, you redirect the browser to Page2.

Method 2: Post + Redirect to self

This is a common technique on forums. Form on Page1 posts the data to Page2, Page2 processes the data and does what needs to be done, and then it does a HTTP redirect on itself. This way the last "action" the browser remembers is a simple GET on page2, so the form is not being resubmitted upon F5.