Pattern 61: Track multiple identical requests
AKA:

You are designing a site that requires users to complete forms or follow links. You understand the need to provide FEEDBACK (41) and THE RHETORIC OF ARRIVAL AND DEPARTURE (20). You want to give users a SENSE OF PROGRESS (48).

People need to make submissions on the web. BUT... they can click twice on a link by mistake, because of lack of feedback or because the connexion is too slow.

Therefore

Always use the same URL to refer to a particular page. Keep track of requests. Ensure that none of your pages contain duplicate information. On commercial sites trap multiple requests either on the server side or using an applet (if they are identical).

This pattern is terminal within this language.

Contributors and sources
Andy Harbach, Jari Worsley, Nielsen (2000)

We are trying to make life easier for the user. One way to irritate a visitor to your site is to let her click on two apparently different links and get to the same page. The browser changed the colour of the first link, so she knew she had been there already. Unfortunately the same page was accessed via a different URL, so the browser couldn’t know that it was the same page. She decided not to visit the site again.

Slow connexions also lead to multiple clicks. A browser back button may lead to a script being executed twice. A request is submitted several times.

Examples
1. Register for a bank account
2. Get confirmation page
3. Hit browser or page ‘back’ button
4. Be asked to ‘re-submit’
5. ‘Yes’ leads to ‘account already taken’

For an internet payment service:
1. Fill in submission details
2. Click on submit
3. While waiting for a response, click submit again
4. Credit card is debited twice.

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