Clone into ext/phpbbde/denyreply:
git clone https://github.com/phpbb-de/phpbb-ext-deny-double-post ext/phpbbde/denyreply
Go to "ACP" > "Customise" > "Extensions" and enable the "phpBB.de Denyreply" extension.
If you find a bug, please report it on https://github.com/phpbb-de/phpbb-ext-deny-double-post
We use automated unit tests including functional tests to prevent regressions. Check out our travis build below: