Control D to duplicate selected text
When text is selected, Control D would create a copy of that text. If no text is selected, the current line of the cursor will be duplicated.
Much like the behavior of Excel to carry down cell values (and a popular Visual Studio plugin), this will allow users to keep the text on their clipboard to paste it over part of the SQL that was just duplicated (like the contents of a where clause).
This works great SSMS! Be nice if it worked in VS as well.
Why can't you just use block editing for this?