PR# 19970 Conflict in once's keys

When we type the following routine in EiffelStudio's editor:

```eiffel
	f
		once ("OBJECT", "THREAD")

		end
```

and then save, the `Features` list panel is emptied, and no syntax error message appears in the Error list panel.
It's only when we click on the Compile button that we get the VVOK-1 error message.

Despite this error, the class text is clickable, so nothing should prevent the Features list panel from being populated.

It's the first time that I see an empty Features list panel in EiffelStudio after saving the class text with no message in the Error list panel to give a clue about why it's empty.

--
Eric Bezault