I navigate a lot on GitHub, in commits for example. But when navigating back, your "GitHub download zip" userscript appends another download button.

Steps to reproduce:

1. Open commits: https://github.com/Mottie/GitHub-userscripts/commits/master
2. Navigate to an older page.
3. Go back to the previous page by using the back button in your browser.
4. See a second download button appear.