I'm running XCode 3.2, and when I double click on a file in Groups and Files it always opens in a new window. What I'd like is for it to appear in the main Xcode editor. I've tried changes the preferences in the Xcode preference panel without any success.
If you using Xcode as a new project there should be a button on the right side of the editor toolbar that toggles between Grouped and Ungrouped. When you click that, and open a new file, it will use the same editor window instead of creating a new window for each file.
If you not using Xcode as a new project, and just the editor, trying single clicking.