If a shortcut is set by Firefox, we prevent setting it for an extension, something similar should happen for shortcuts that are already set by an extension.
- Assign a shortcut to something (like Ctrl+Alt+S)
- Try to assign another shortcut to the same thing (Ctrl+Alt+S)
Expected results: An error is shown, stating that this shortcut is already defined (possibly including at least one extension name that already defines it)
Actual results: The shortcut is set, but it may or may not execute the related command