If you are working with a FORM (form = any time a web page presents you with fields to enter data and then it saves that data when you Submit/Enter when your done) and submitted it (committed the data) and THEN you click on the browser's BACK BUTTON ( Safari's BACK BUTTON, as an example) then you very well may be presented with a "RESUBMISSION" message similar to this example:
[image: 'Confirm Form Resubmission' message]
This is not an error. This is the browser telling you that you already submitted the form and by action of clicking the "Go Back" button, you are essentially trying to submit the form again (resubmit).
To avoid this message you would advance through whatever page you are interacting with and or use the websites internal navigation features (such as the sites menu (not the browsers's menu) to get to where you want to go.