If we are renaming a sheet that exists only once in the schematic, then
we don't need to warn about case sensitivity.  If the sheet exists
multiple times, we do.  We can't check this using SCREENS because the
screens don't maintain full state information about where they are used.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/17901

(cherry picked from commit ba497474)