I'm having no problem adding components to to the Palette in Delphi 2007. Generally I do this by adding units to a .dproj and then installing the .dproj. I've tried to delete a unit - and that appeared to work. Later I tried to add the unit back into the .dproj and I received an error message that the unit was already there - even though not visible. Can anyone explain what I've done wrong, and the fix?

That's because you never actually uninstalled it. Press F1 and index "Installing Component Packages" to learn more.

In a nutshell, there are two things you must watch for: the Delphi Library Path, and the available component list. Things on the component palette are chosen from the available component list, but not every available component need be on the Component Palette.

Also, never compile a component into the Delphi Component Library Package, unless, you are absolutely certain that it is bug-free and completely finished. Even so, don't do it.

Hope this helps.