Comment 3 for bug 350571

I hope you'll hear me out, as I have some thoughts I hope will help:

JavaScript comes to mind as a solution - when any module is set to display in a specific position, remove the position from the other boxes. (In reality, it would be more reliable to just reconstruct the menus for each one after each change).

I would also suggest changing from drop-down menus to radio buttons, or a custom system (perhaps somewhat similar to the system used for rearranging the menu) to help prevent confusion - it's very disconcerting to see items in a drop-down menu disappear. This would still require JavaScript to implement.

If client-side scripting is to be avoided, there's certainly the option of validating the choices server-side.

Time permitting, I would be willing to write the JS to ensure that this won't happen in 4.7.