If you hold down the Option key while drag & dropping text within the same window, you will get a duplicate of the selected text and not a cut & paste.
It started out as an experiment in Mail.app (formatting an email for reply), and so I tried it in Script Editor, TextEdit, Tex-Edit Plus, and Microsoft Word -- it worked in all of them. So if you need a quick duplicate of a section of text, just Option drag and drop it.
[robg adds: This makes sense, as the Option key in the Finder is the "duplicate" modifier. I had just never though to try it with a drag and dropped text selection...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050116024215578