Actually that is not a plugin but rather is part of the core itself. To use that feature you need to navigate to admin/posting and enable the "Check for similar questions when asking" setting.
Welcome to the Q&A site for Question2Answer.
If you have a question about Q2A, please ask here, in English.
To report a bug, please create a new issue on Github or ask a question here with the bug tag.
If you just want to try Q2A, please use the demo site.
April 20, 2021: Q2A 1.8.6 released!