It can be done from the 'issue/show' view or from the context menu on the issue list.
The Copy functionality is of course only available if the user is allowed to create an issue.
It copies the issue attributes and the custom fields values.

git-svn-id: http://redmine.rubyforge.org/svn/trunk@873 e93f8b46-1217-0410-a6f0-8f06a7374b81