Excellent.  Thanks for volunteering.  Here is a feature request for you to get started

http://askbot.org/en/question/11506/rfe-when-marking-as-duplicate-let-users-specify-the-duplicate-question/

The source code is at

https://github.com/ASKBOT/askbot-devel/tree/master/askbot

The usual method of working is just fork it in github and send in a pull request referencing the above question.  If you have any questions, feel free to ping me offlist

Rahul