title: PHP clear browser cache to avoid form data resend after refresh
body: <p>I'm developing a PHP-MySQL app that enables registered users to enter text comments.  Problem is:</p>
  
  <ol>
  <li>User sign-in into the web site - OKAY</li>
  <li>User presented with a form to submit text comment - OKAY</li>
  <li>User enters text comment and submits - OKAY</li>
  <li>I have a routine that sanitize the comment &amp; save it into MySQL (with userid, textcomment, datetime stamp) &amp; present back the user that his/her comment is entered - OKAY</li>
  <li>User decides to refresh browser - a duplicate comment is entered - BAD!</li>
  </ol>
  
  <p>I was thinking 3 options:</p>
  
  <ul>
  <li>OPTION 1: Routine that checks: last time user posted comment, and if so, check if this is a duplicate. If duplicate then display error message.</li>
  <li>OPTION 2: Routine that does not allow a user to post too quickly. So basically do not allow postings of comments within 1 minute or so. So if browser is refreshed the comment will be ignored.</li>
  <li>OPTION 3: Manipulate the browser cache to clear out its contents so when refreshed no duplicate will be entered.</li>
  </ul>
  
  <p>Now in the context of my application, my concerns with OPTION 1 and OPTION 2 is performance PHP-MySQL since I already have various queries within the same page that push/get data from databases. So OPTION 3 may target the issue differently.</p>
  
  <p>Questions is: If I go for OPTION 3 can this be considered a Best Practice? meaning clearing the browser cache is the best most effective solution? I have read that there are consequences too? your thoughts are appreciated!</p>